Now, in all honesty when you analyze units, you can't just math-hammer them and say "Well, this unit is more effective at killing XYZ than that unit." Sometimes, you have to take a look at what a unit does for you, or to the opponent, and it's not always 'kill lots of things.' I'll start with a few simple examples.
The Rhino (and other Transports)
It's 35 points. It has a storm bolter. How often does a lone storm bolter do something more spectacular than take out the odd infantry straggler? The Rhino doesn't kill things. It might tank-shock someone and kill some moron trying Death or Glory.
So, how come you see people taking them? This one's the 'gimme' example. The Rhino (and any transport) keeps your infantry from eating small-arms fire from the get-go, and gets them places faster. For 205 points, I can take 10 tactical marines with a flamer and multi-melta, and get them into position to use them. I can drive up, pop smoke, and move up again and make the enemy worry about a multi-melta near their lines. Or, I can tank-shock infantry, pile out of my box and flamer/rapid-fire my new friends. Or, I can take a shorter tank-shock and do the same thing.
All that for 35? That's a bargain.
Kroot (and other 'bubble wrap')
I pay around 120 points for 10 Kroot and 7 Hounds. How often do they kill stuff? Well, there was that one time they took out a chaos Raptor squad... But beyond that, no, they really DON'T kill stuff. They fold like wet paper sacks in hand-to-hand, they die to flamers in a comedic and terrible fashion, and can kind of rapid-fire stuff. Kind of.
So, why do I take them?
It's called a 'meat shield.' Bubble wrap is another term I've heard, but I prefer meat shield since I've bought lunch meat that comes in tupperware containers explicitly for carting around heaps of plastic infantry that have a generous amount of clear coat on them. Plus, it's good for a laugh when you put your meat shield back into a meat container.
Case in point today against Tyranids: A Trygon pops up near my lines. There are a heap of Kroot between it and juicy targets like a Hammerhead, Broadside Team, and a couple of Crisis Suit teams. Had I not taken the Kroot, I would have had ONE turn to kill the Trygon (which is not necessarily a given, with 6 wounds at T6 with a 3+, even with plasma rifles) before I lost something important. Instead, I took off four wounds with a heap of gunfire, lost a Piranha to it, and then had a Crisis Suit squad go up to BS5 from markerlights and nuke it.
Against meltaguns and the like, you have to do something like tank shock the kroot out of the way to deploy within 6" and get your crucial 2d6 penetration. S8 on AV13 is not a gimme by any stretch of the imagination, but S8+2d6 is a Hammerhead in trouble. It does stop annoying alpha-strike tactics like scout-moving a Vendetta up, and meltagun+meltabomb rushing a Hammerhead, provided you space out and deploy properly.
The Piranha (and other fast, generally-expendable threats)
Here's another short story abou Tau and sacrificing units for the greater good. I field two squads of two Piranhas; they pack Fusion blasters, BS4, and one mounts a Disruption Pod while the other carries the wargear that lets me split fire.
So, one of my buddies was getting into Black Templar, so he's rolling a pair of Land Raider Crusaders with furious charge lightning claw terminators. Ouch. Well, we set up in table quarters, so I rush my Piranhas up in front of his Raiders. He HAS to deal with them; there is a very real chance I can trade 300 points of my army for 500+ of his, and force his terminators to run at me. Considering plasma rifles and markerlights are involved, this doesn't bode well for the Terminators.
So, he shoots at them a little, and I get my shots at his Raiders. Four meltaguns and three railguns later, I've managed to...shake...a single Land Raider. Even with smoke, you'd think all the fething AP1 fire might do some damage. Nope. Sorry. HOWEVER, since these are vehicles, he can't tank shock through them. At this range, he can get an S4-5 ram off on AV11 with a 3+ dodge save. If he does NOT halt and engage them, they'll simply try again next turn. So, he spends a turn or two repositioning and brutally gunning them down.
The Piranhas did not kill a bloody thing.
The Piranhas cost him two turns of movement.
The Piranhas dictated his game, and in turn gave me the time I needed to immobilize one Raider and kill the other.
My Point on Meat Shields and Other Expendables
Sometimes, just being there is enough. Cheap infantry can buy you a turn against assaulty armies, and cheap, fast vehicles that are a credible threat (IE: Multi-melta/Heavy flamer land speeders as well as Piranhas) HAVE to be killed. It's the fact that you can pay points and troops for time, or make them dictate your opponent's movements that makes them worth it.
Rambo, Deathleaper, Doom of Malant'i, and Company
Another category of 'doesn't make points back' is the type of unit that makes you change your deployment and target priority.
Let's be honest: Rambo doesn't often kill much. Marbo appears, chucks a demolition charge, and draws some fire. If you left the opening, Rambo chucked it at the back of a vehicle or at some infantry that weren't in cover. I mean, some good deployment counters that. Additionally, if you spread out in terrain, you minimize the places that Marbo can actually be placed.
Deathleaper and the Doom fall into the same category (though the Doom kind of needs some clarification on its Psychic Vampire crap. At least it's not sparkly like diamonds, right? Right). Worst comes to worst, the Doom kills some infantry, and then you hit it with S8 until it fails a save. Doesn't matter how many wounds at has if it suffers Instant Death.
Outflankers: Or, sometimes the bluff isn't worth it
Now, another category came up from an onlooker. He talked about "Man, you have a hammerhead right on the flank. Some genestealers would be nasty, there."
Not really. First, I can deploy away from the flanks. Oooh. Ahh. It's not THAT hard to get away from foot infantry. Second, in this particular situation, the Hammerhead had moved 12 inches. Had 8 genestealers actually charged it, they might have gotten ONE penetrating hit. (8 stealers = 24 attacks; 6's to hit means 4 hits, which means MAYBE one penetrating hit). Oh. Then the Hammerhead zooms away 12", takes a shot, and some of the rest of my army nukes the stealers, or weakens them sufficiently for me to ignore them.
Ok, outflanking CAN be annoying. On the other hand, there are some problems with it.
1) Reserves. Did you show up on time?
2) Entry Point. Did you show up where I needed you?
3) Impact. Did you do something useful when you showed up?
See, with Khan, I could theoretically outflank a Land Raider Crusader (provided I take it as a dedicated transport) full of assaulty goodness. I could also not see it 'til turn 4-5 and have it arrive on the other end of the table, and there's 450+ points that did not actually KILL anything.
Admittedly, some armies have reserves manipulation and outflank entry manipulation (IE: IG command squads). That helps to an extent, but you can still use things like, oh, meat shields to work around it.
This isn't to say that outflank is ALWAYS useless. However, if you want the enemy to pay attention to your wildcard threat, it should be less of a threat and more of a promise.
When you're analyzing a unit, lethality isn't everything. You have to look at the army as a whole; what can each part bring to the table and what does the sum of the parts do for you? If you're going to play expendable units, make sure they're expended usefully. I mean, stuff's gonna die, so get the most out of it and make it work for you. If you're going to bring a bluff card, make sure it's something the enemy HAS to take seriously and make sure it's a somewhat reliable bluff card.