Obviously, I will continue to discuss Allies individually alongside each of the Armies in Focus articles, but I felt it worthwhile to compile thoughts on each into one concise article for easy reference. We're all still getting used to the concept afterall, and it can't hurt to have things all in the same place.
So, Allies form the same structure every time - 1 HQ, 1-2 Troops, 0-1 Elites, 0-1 Fast Attack, 0-1 Heavy Support.
The most obvious reason to take Allies then, apart from purely aesthetic or thematic reasons, is to get yourself an extra Troops choice, or a different kind of Troops choice.
Identifying what exactly it is you require or desire from an ally is the first step in the process.
For example - Blood Angels are (traditionally) very good at having mobile, aggressive Troops units, such as Melta hunter teams. The kind of Troop they'd be looking into getting would therefore be more static, probably a more resilient firepower unit, such as Deathwing Terminators, Space Marine Scouts with Sergeant Telion, or an Imperial Guard Platoon. Things that overcome a specific flaw in their listbuilding options.
In contrast, Eldar have mobile but relatively weak Troops, the resilience of which is entirely predicated on the AV12 and Energy Fields protecting them. They are more likely to desire more solid and deadly units to be expendable in taking the fire otherwise likely to annihilate their T3 bodies. Deathwing again can fill this role, or perhaps Grey Knight Purifiers.
Not every alliance is founded on the requirement for new or better Troops, however.
Some alliances are predicated on the neccesitation of other factors, such as:
Ranged Support Elements (ie, Railguns)
Close Combatty Stuffs (rarely worthwhile, but not unlikely most players will consider it)
Different Threat Bands.
These are largely self-explanatory, but I'll cover each for the sake of completeness:
Some armies don't get access to these, the most deadly and reliable anti-tank weapons in the game. Most armies do, granted, but most are Imperial or pseudo-Imperial. Those that don't, such as Necrons, Orks, or Chaos Daemons, now have the opportunity of selecting a nice Ally of Convenience to fill that niche for them - sometimes they Desperate Allies instead (such as if Orks chose Space Wolves) but that's rarely necessary.
It isn't QUITE accurate to say that every army NEEDS Meltaguns, but now every army bar Nids has easy access to them, so it's pretty difficult to justify NOT taking any.
Different Threat Bands -
Some armies largely function within a limited series of ranges. Orks, for instance, have only really Lootas to fire beyond the 24" mark. Sometimes, an Ally might be chosen for their capacity to fire further than this to put more and differing pressure on enemy forces (such as forcing them to advance into your other weaponry.) Conversely, you can choose deliberately shorter ranged allies, to keep the foe further back. A good example of this is Imperial Guard choosing to ally with Space Wolves - Grey Hunters being shorter ranged than varied artillery pieces and Heavy Weapon Teams.
Not everyone gets something with Skyfire, and especially not the opportunity to exploit their enemies not having it. A good example here is Orks with Necron allies, or Eldar picking up a Razorwing or Voidraven. Vendettas and Hydras are also splashable in most forces. Again, not having a Flyer doesn't mean you auto-lose, but failure to adequately deal with them means that unbalanced Flyer lists can destroy you unreasonably easily.
Psychic Defence -
If you consider Deny the Witch a good way to prevent enemy Psykers from doing what they like you need your head examined...or perhaps you use loaded dice. Either way, get over it. Like above with Meltaguns and Flyers, you need a way to deal with these things (Psychic Powers) or get destroyed by unbalanced eggbasket lists. With Psychic Hoods barely worth the cloth they're made of, look to Runes of Warding and Runic Weapons for REAL protection from enemy casters, remembering that if you aren't Battle Brothers, they will interfere with YOU too (take heed, Grey Knights...)
Ranged Support Elements/Close Combatty Stuffs -
These two are the same side of different coins. Orks lack ranged anti-tank, and so quite gleefully take Guard to provide such things. Guard lack close combat tarpit units and more importantly (not really, but occasionally it's true) genuine melee-capable units that will win fights for them, like Nobs or Terminators.
Sometimes you get lucky and can provide both in the same unit, ie Deathwing.
Once you know WHAT you want from Allies, it's time to look at page 113's Allies Matrix (not as interesting or groundbreaking as the film) to see WHO you can have.
Allies are divided into 4 basic categories, although the 4th is a misnomer as it indicates the Alliance is prohibited. These are;
Battle Brothers - the best kind of Alliance. If you can get what you want from these guys, take it, no ifs, not buts, no 'but I prefer the idea of using x other race' - don't be a dumbass. The rules are designed to punish stupidity (but not fluff-related stupidity, as I will illustrate in another article.)
Allies of Convenience - This is the fall-back option. Sadly, some armies (Daemons, CSM, Orks, Necrons, GKs, DE, Eldar) have limited, or no Battle Brother options, and are forced to look to the Allies that contest their own Objectives if too close. :-/ 'WTF Ally, Y U Teamkill?' aside, these are better options than to have none (*cough Nids cough*)
Desperate Allies - Never use these. I've played with Wraithsight, and fuck you too GW. There is very little more frustrating in a toy soldier game than your toy soldiers not doing what they are supposed to do, when ANY OTHER UNIT IN THE GAME could have done it instead, and better. Yes, sniping characters isn't easy, and for Telion/Vindicares it's arguably harder than before - but Warlocks getting sniped can grind a Wraith army to a halt based entirely on a fluke result - and that kind of thing is Just No Fun. Having this apply to EVERY unit in your army within 6" of an Ally, including the Allies of course, is just asking for frustration on a grand scale. Rolling a 1 carries a sense of disappointment irrespective of situation, but when potentially game-breaking, it's moronic as a system for a game that CLAIMS to put 'fun' over other considerations (balance, for one...)
There you have it. You know what you want from your Allies, you know who to turn to to get it. It's a ploy and a marketing tool to get you to go out and buy more Codexes, and hopefully a few boxes,and then why not start the army proper, you're half-way there already? Etc, etc.
It will act to prevent a lot of the stagnation people complained of at the end of 5th (actually, from the middle onwards really) but that's symptomatic of poor rules-writing and a lack of good inter-faction balance, not anything TRULY to do with the possibilities of Allies.
The next article on this topic will hopefully amuse you as much as it has amused me this morning researching it...
If the guys from Penny Arcade want me to take down the comic I blatantly stole for my image on this post (I love this one, definitely one of my favourites) then no biggie, consider it done. Hopefully, they'll appreciate the decision instead. :)