I see a lot of stuff in lists from time to time, and sometimes the gear/powers/extras just don't seem worth it. At other times, investing a few more points can and will make the difference between a unit being crap and a unit being worth it. Sometimes the analysis is as simple as math, but most of the time it's more subjective than objective in evaluation criteria.
Versatility is almost always a good reason (or excuse) to add more gear. If you can drop a few more points and get a useful capability, then do so. If you can get a multi-role unit, then you increase your chances of being able to get some use out of it.
Space Marine Land Speeder w/ Multi-melta, Heavy Flamer
Speeders start at 50 points with a heavy bolter. For another 10 points, I can give it a multi-melta, and let it hunt vehicles. For 10 more points, I can give it a heavy flamer and the ability to roast infantry. Now, given it's an AV10 vehicle and packing short-ranged weapons, it's not so likely that it will use either ability at the same time. On the other hand, for ten points more on the second weapon, you've made the vehicle a threat to everything on the board. You'll probably only use one of the weapons in a game, but it's simply a utility unit.
Terminators with a Cyclone Missile Launcher
So, you're rolling a footslogging regular terminator squad out of the vanilla 'dex. If you drop 30 points on a Cyclone Missile Launcher, you've given the unit the ability to engage light armor at a distance with two S8 shots, and complimented its ability to direct small-arms fire onto softer targets. It's 30 points well spent.
Howling Banshees with an Exarch w/ Executioner, and maybe other abilities
Howling Banshees are all S3 maniacs with power weapons. This, and doom, are sometimes adequate for dealing with well-armored infantry. On the other hand, charging into a monstrous creature with Banshees is kind of hoping for a lotta 6's, and they can't scratch a T7 monster. Now, if you drop the points for an Executioner, you give the squad a little more efficiency in killing the light stuff, but you can also threaten to finish off wounded monsters and even scratch vehicles every now and then. It's definitely worth it.
Other times, you CAN give a unit an extra capability for some points, but you don't always need to. The best example I can some up with for a situational versatility upgrade is the Eldar Fire Dragon Exarch. For 17 points you can upgrade a Dragon to an Exarch with a heavy flamer that re-rolls failed wounds.
Cool, huh? Well, sometimes. Fire Dragons will only ever usually blow up monstrous creatures and vehicles, but if you need a little extra anti-infantry firepower, then they're a place to pick it up. Now, if you've got Storm Guardians and/or a Seer Council, you've got some template weapons already in your army. However, if you don't, then Fire Dragons can be made to fill that niche. In cases like this, it's best to look at the rest of your army and see if you really do need to spend those points.
Just Getting The Tools
Other times, you'll be taking some upgrades just to get your unit to do what you want it to do. Psykers, for example, tend to feature some upgrade lovin' just because you have to buy powers, and may want to pick up some armor. Other units simply don't work that well when done in a minimal nature. And sometimes? A few extra points just gives the unit the ability to actually DO something.
18 points for 4 attacks on the charge. 22 points for four rending attacks on the charge. Not a brain-teaser to figure out that in this case, you really ARE better off giving the unit some teeth with the Kisses.
Farseers start at 55 points, and it's pretty easy to get them up to 150 or more. They're your only source of psychic defense in the codex, so you're going to drop some points on Runes of Warding. Next, you're going to take at least one psychic power, and likely two, which is another 60 points minimum. Then, maybe a bike, maybe not. Why spend all this? It's what you have to do to get mileage out of them.
Now, you could just buy a heap of Nobs. On the other hand, you could add a painboy (for Feel No Pain joy) and some Power Klaws (so you can actually do some damage to non-infantry targets). The unit gets pricy, but the capabilities improve, and comparing a naked nob squad to one about the same poitns, but kitted out, is no real comparison at all.
In this situation, they're the workhorse unit of your army. You really NEED to give them teeth; an assault cannon is their best option for harming anything storm bolters can't handle, and even a lone chainfist makes a difference against larger armored targets. The unit jumps from 215 to 250 with just one of each, but it makes the difference.
...do you really need that?
Sometimes, well, units just don't need gear. They're just not going to use it, or not really benefit from it. Sometimes it's a suicide unit, and sometimes it's gear that's just extra in terms of their intended use.
Some examples include...
Imperial Guard Melta-vets
10 guys get out of a Chimera, and blow up your land raider. The punch line? You're going to kill them next turn. If I'm the IG guy, I spent 30 points on a melta and 55 on the chimera; it's the bare minimum. Suicide units do not need neat little extras. If the unit's expendable, make sure it doesn't cost too much to expend.
Tactical Squad Sergeants with Power Weapons/Fists/Combi-Bolters
Admittedly, sometimes they can be useful; everyone's got a story. On the other hand? You're 9 attacks and 2 fist attacks/3 power weapon attacks. Do you really think that a dedicated melee squad can't roll you?
Same goes for combi-weapons, generally. Most of the time it's filler. Oooh, ahh, one extra shot. I admit that in HQ units, a combi-weapon can be a decent use of their BS or something, but most of the time single-use gear is simply filler you toss in if you have a few odd leftover points.
Ask yourself if the gear's going to be used reliably. Some times, it is. If you intend to, for whatever reason, leave a tac squad camping on an objective, then maybe a fist isn't a bad idea since it'll let you watch out for yourselves. But, if you're going to rapid-fire out of a squad? Don't do it.
Extraneous Exarch/whatever Powers
Bladestorm, for the Eldar, is debatable. However, in larger point games, trying to turn them into a tarpit/mleee threat with Defend/power weapon/shimmershield? Maaaybe in smaller games. Maybe. As yourself if you really need it, if you're really going to do it.
Right Unit for the Right Job?
Most of the time, you can get several ways to fill a specific role for your codex. Want to kill troops? You got a lotta ways (well, in the better-designed codices with a variety of units, that is). So, you start sitting down, and asking yourself how you want to accomplish your goals.
As a refresher, your most basic goals/capabilities for an army would be, in no particular order:
1) Objectives: claim, and contest
2) Kill Vehicles
3) Kill Infantry
This is, honestly, the most codex-specific part of this article. It really, honestly depends on what you play as to how you do this. It's a bit of an art at times. You look at your units, and decide what tradeoffs you want and are willing to take.
If 2+ units compete for a task? Sit down and compare them.
1) Can they all do the job reliably?
In the linked example, Stelek discusses the use of sacrifical melta-bearers in an IG army. I'm going to take a slightly different tac, and look at the availability of flamer-type weapons in an Eldar army. I can come up with four options:
Seer Council on Foot Doom Farseer w/ 4x Destructor-Locks 
Biker Seer Council 
Farseer w/ Fortune/Doom/Bike/Runes of Warding
1x Embolden Lock
2x Schmuck Locks
10 Guardians, 2 Flamers, Destructor Warlock 
6 Dragons; one is Exarch w/ DB Flamer, Crack Shot 
Can they all do the job? More or less, yes. We have re-rolls on four heavy flamers, we have re-rolls on four heavy flamers on bikes, we have two flamers and a heavy flamer, and we have a single heavy flamer w/ re-rolls to wound. Template burny death? Check.
2) What's the cost of taking the unit?
Here's where we get a little more complex. It's not always as simple as points. Ask yourself:
a) How many points am I paying, and for what exactly?
b) What will it cost to actually employ the unit, IE do I need a transport/other kind of support?
c) How easy is it to employ the complete capability?
d) What Force Org slots am I using, and if it's a 1-way ride can I afford to lose the unit in question?
The foot council costs ~200 + 100-145 for a transport, and an HQ slot. Stop the vehicle and you stop the capability, and they're purely sacrifical. However, I need another Farseer if I want anything other than Doom, and that for very long.
The Biker Seer Council is hellishly expensive, BUT provides other capabilities. However, its cost dictates my build. It requires psychic defense and/or a whole lotta gunfire to kill, or very hardcore melee units to dispatch. Losing it early will likely cost me the game.
The Storm Guardians are relatively cheap, even after the 100-145pt Wave Serpent. They're not bad at the job, but you probably want Doom support for them. They're also unlikely to survive the trip, but if you were relying on Storm Guardians to hold objectives I have to ask why you took them out of the transport. It's not that they're suicide troops by nature; it's that they're not hard to kill with small arms fire, or anything else for that matter.
The Fire Dragons are also realtively cheap, but this isn't their primary mission. For them, it's an extra capability. We will likely NOT be using this neato extra, if there are larger tanks to pop. And, of course, the transport is mandatory for these guys.
My evaulation for the example?
Not really sold on the Burny Foot Council. It's a lot of points, and will kill one target really, really dead.
Storm Guardians are a decent addition to any given Eldar mech army. They set stuff on fire, and are reasonably cheap.
Fire Dragons? It's not the first stop for the flamethrower, but if you have points for it and could use the extra anti-infantry power, then it's 33 points more for something you were going to take anyway.
The Seer Council dictates your build. Pure, and simple. Pick it if you're playing a Seer Council army, obviously...
3) Can I slot it into the army and strategy?
This is something you, the general, will have to ask yourself. Most people have a theme/strategy in mind with their army. Your codex usually has some say on this, as do the rules of the game (hint: vehicles usually good, cover good), and your own personal preferences. This is what it is to be a player and a general; understanding where stuff slots into your list, or if something dictates it and becomes your theme.
Whatever you do, focus on something, even if that focus is in fact versatility. For Space Marines, honestly, versatility IS the theme, and lots of other armies should at least consider it.
Continuing the example...
If I'm trying to do an all-foot Eldar list (...for whatever reason) then putting one unit in a tank is foolish; it's going to get all the anti-tank fire. Hell, this goes for any list. Making something the odd-man-out is a sure-fire way to get all the enemy's anti-odd-man-out capabilities pumped into what'll soon be odd-chunks-over-there. The Seer Council certainly fits this bill of 'lightning rod', but with Fortune on and other threats, it's usually solid.
The rest of our happy little burny elves in the example are generally small enough to be a component. They all lend themselves to mechanization, but that's kind of an Eldar trend. Have good tanks? Take good tanks. Cry when someone pops a holo-fielded Falcon on the first shot. (Hint: odds of getting a 'dead' result on a Falcon are 1 in 12. Crying is permissable if it happens on the first hit).
Ultimately, I believe that it's not hard to get some efficiency into your builds, but it's difficult to learn the art of trimming the fat from your army after a point. Sure, a unit screws up or doesn't perform as you like for your strategy, you cut it. You start settling into a build, but...there's almost always tweaks to it. When it comes to change, there's three types: good change, bad change, and change for the sake of change.
I can but offer you some waypoints on the road to getting a better army:
1) Can it do what you want it to?
2) What does the capability really cost?
3) Can I use it in this army? I think that #2 is the most difficult part.
And the rest...is on you, commenters and readers.
EDIT1: Briefly tried breaking this down into mutliple posts; lost a heap of formatting. I'm keeping an open mind towards breaking articles into parts, but this one might stand as a wall of text for a while. If I can figure out how the hell to do cuts, I might consider that.