I'll wrap up my thoughts on the Deathwing's ally options, and make it the third (and probably last) article on the theme.
First, though, good news on my front: employment's finally here. On the bright side, it means I now have a two-way cash flow, and can afford new models. I mean, my opponents know that my Tau battlesuits have looked a whole lot like Eldar jetbikes, for example...
On the downside, combined with grad school this semester it means less time in the hobby, so there might be a little less blog posting. Eh, it's life. On to what you came here for, though: The Deathwing.
Now, part of the issue of taking allies is that it's points not spent on the Deathwing. This should be obvious, but it bears mentioning: the more you spend on allies, the less you're spending on Deathwing-only options.
With that in mind, the goal of allies is to bring something you can't get otherwise with Deathwing options. Here are some things you simply cannot take with just Deathwing troops, and/or even Dark Angel Space Marines.
1) 2x Melta in a troops selection, or even readily delivered
-ISTs out of either codex
-Sisters of Battle as troops
-Sister Fast Attack troopers
2) Psychic Defense (a Ld9 hood does NOT cut it, nor does Ezekial
-Grey Knight Hero
3) Better odds at turn one
-DH Inquisitor with Tarot
4) Deep Strike Defense
-DH Inquisitor w/ Mystics
5) More Armor
-Rhinos (either side)
-Immolators (max of 2, from SoB Elite and Fast Attack selections)
-Chimeras (either, if a bit pricy)
(Note all the armor here is like yours with the smoke launchers; glance-only. Aw, Yeah)
6) Power Weapons that strike at initiative
-Grey Knight Terminators. (Mmm, S6 power weapons)
Honest Appraisal of Allies
I have to favor the Daemonhunters codex for allies. You get the same ISTs, but you get an Inquisitor with better options. (Note: I'm talking about the HQ inquisitors, not the piddly elites. However, in a pinch, you can get an elite DH inquisitor with Tarot for 35. Minimal commitment! Maybe spare him 2 points for pistol/ccw for some love...)
The real big thing from the allies is the psychic defense. There are some powers that aren't that frightening, but Lash + Oblits = Dead Terminators, and since we want to make Termiantors effective...yeah. At a minimum, I would think long and hard about dropping the 167 for Hood/Tarot Lord, 3 Heavy Bolters, and 2 sages. It's cheap, and it brings an amazing amount of utility for the cost.
The second consideration would be melta troopers. 128 gets you 5 interpeid, sacrifical storm troopers with meltaguns and a kitted out Rhino. If you want to commit to more AV12, you can get Chimeras, but I would have to test and see whether I feel like more AV12 is worth the cost.
If you check out the sample lists in my other posts, you'll note they include 2 Dreads, 2 Chimeras, and 2 Predators. That way there's four AV12 units, and two AV13 units. All the AV12 can threaten you with melta, and the AV13 can threaten the lighter armor at range. My gut suggests that if the enemy could KO your meltagun transports (and it's not that hard to drop 5 guys with T3 and a 4+). I'd lean towards Rhinos for Sisters, or Chimeras for the guardsmen. The Sisters are a bit more likely to survive on their own, with decent leadership, numbers, and power armor.
At any rate, the main goal is to help your terminators get to the meaty insides, or in a pinch beat down the vehicles that survived. Plus, they'll be durable.
Much more than the basics, and you're cutting into your Troops/Terminator budget. This means unless you're going for Grey Knight Termies with the hood, or a Grand Master, you should probably skip them. Admittedly, a Grand Master with the old-school 'lose all your wounds' Force Weapon can do some serious damage to whatever you point it at, and he gets you the Ld10 hood.
5-man IST teams with 2x melta, probably in a Chimera 
10-man, 2-melta Sister team; Sister Superior w/ Book of St. Lucius in a Rhino 
OH Lord w/ 3x HB, Chururigeon, Hood 
DH Lord w/ 3x HB, 2x Mystics, Tarot, Hood 
The basic Deathwing loadout is 630 for Belial, and two DW squads with assault cannons and chainfists. Add in 167 for a DH Inquisitor Lord, and you've got plenty of slots left, and you've spent 797. With the 300-360 on melta allies, you're moving towards an armored build that's around 1,000 points; there's plenty of room to get some backup.
I feel like, overall, it's a reasonably strong build. Is it perfect? No. You're using a marine codex that's been obsoleted by the newer one, and ultimately it's a theme build with Terminators, which has its own problems. However, I think you can get some serious mileage out of the allies. I think it's worth it, and I think I've talked myself into rolling with allies at this point.