More than any other single aspect of the upcoming 6th edition Warhammer 40,000 ruleset, the new Allies rules has people up in arms. It's managed to unite both hardcore and fluffy gamers, two groups that rarely agree on what colour the sky is, and has even managed to overshadow random charge distances, terrain that has a random deadly attribute, and shots at flyers needing 6's to hit. This is no mean feat, but it's also pretty heavily misplaced.
For the hardcore gamers, the complaints have largely centred around how 'broken' this will make the game. A Dreadknight in every codex! But most of these people seem to have forgotten two very fundamental points; Allies units still cost points, and you have to bring an HQ and a Troop unit before you can get access to a second Troop, and one each of Elites, Fast Attack and Heavy Support. For a non-Grey Knight army to take a Dreadknight requires, not just finding the hundred and thirty points for a naked Dreadknight, but another hundred points for a Strike Squad, and twenty-five points for a bare-bones Inquisitor. And for a grand total of two-hundred and fifty-five points, you have a T3/4+ Inquisitor with no decent gear, five power armoured Grey Knights with no upgrades, and a Dreadknight with no guns and none of the mobility needed to get it right into close combat. A steal at twice the price? No, I think not. And none of the other chapters are better, trading slightly cheaper Scouts for significantly more expensive HQ units.
What's much more likely is the use of Allies units to cover over small weaknesses in the individual codexes. In particular, a heavy priority is going to be placed on books that have relatively solid, relatively inexpensive HQ and Troops units, since otherwise you're paying a hefty tax for a single unit from one of the other slots. It's quite likely, then, that Eldar, Blood Angels, Grey Knights and Imperial Guard are about to become every race's best friends; Farseers are powerful pskers with good defences and Pathfinders have the chance to direct AP1 fire at a model of the player's choosing on to-hits of 6, Blood Angels have ASM as Troops, Grey Knights have force swords on their Troops, and the Guard can provide access to cheap, plentiful bodies, with cheap, plentiful special weapons. The other races will probably see use, as well (I've already heard some players plotting to bring Space Marines along to get access to the Stormtalon), but to a somewhat lesser degree, because they lack strong HQ or Troops units, and the Allies system provides only limited access to the other slots. And while a Dark Eldar army with Terminators, or Chaos Daemons with Leman Russ battle tanks might sound terrifying at first, those armies would have to give up a lot from their core competency in order to get a handful of units which can't ride in their transports, benefit from their special rules, and in some cases have to test each turn to see if they can even act. This isn't really a recipe for an overwhelmingly powerful force-multiplier, so much as a way to bring a few small units along to cover a weakness.
As an aside, I've been kicking around the idea of Allies myself. In order to fit a Blood Angels Librarian with a jetpack, five naked Assault Marines, and a Landspeeder with a multimelta and heavy flamer into my 2000 point cadre, I had to drop six Firewarriors, two Piranhas, two bodyguard Fireknife XV8s, and the fusion blaster off my commander. The results certainly aren't bad, but they're not overpowering, and I doubt I'd even try to squeeze a detachment in to a lower points level. You just have to give up too much to get a little.
Now, the other argument against Allies comes from the fluffier player. Their criticism usually runs along the lines that 40K's slogan is 'In the grim darkness of the far future, there is only war!', while Allies seems to make it more 'In the grim darkness of the far future, pretty much everyone is besties, except Tyranids'. Which is just silly, of course, since the only reason these Allies are being fielded is to fight a war, so that element has hardly gone anywhere. Indeed, arguably there's even more war to go around now, since not only are armies fighting their own fights, they're getting dragged into other peoples', too!
In all seriousness, though, the races in 40K have never been as single-mindedly xenocidal as the fandom often portrays them, again, except for the Tyranids. Yes, all things being equal Imperial forces would rather fight against from non-Imperial forces than alongside them, but there've been plenty of instances in which all things weren't equal. The Valhallans fought alongside Eldar against a Chaos incursion (after fighting against the Eldar who didn't bother to explain why they'd appeared on Valhalla). Orks of the Blood Axes tribe are known for being mercenaries, their shootas and choppas for sale to the highest bidder, including in one instance Commander Puretide of the Tau Empire. The Space Wolves and the Eldar combined forces to defeat Grimtusk Bloodboila. Space Marine, Imperial Guard and Tau Empire forces co-operated to fight back a Chaotic incursion in the Firewarrior game, and if people don't consider that fluffy enough, the Ultramarines and the Tau also fought together against a Necron tombworld. The Grey Knights secured Malan'tai from a Keeper of Secrets, and returned the dead Craftworld to the Eldar; not a fight, admittedly, but they could have simply destroyed the defenceless Craftworld, and didn't. The Blood Angels and the Necrons united against a Tyranid incursion. The Dark Eldar have 'helped' both the Tau and the Eldar in their current codex, always for their own ends of course, but they've still fought beside them. Inquisitor Czevak has been allowed inside the Black Library. Yes, everyone is at war with everyone else, but some wars are more important than others; if the Imperium loses a world to the Orks, or the Tau, or the Eldar, they can always try and get it back later, whereas if they lose a world to the Necrons, or the Tyranids, or Chaos, it may well be lost forever. People at war for the fate of their species have to pick their battles very carefully, indeed.
That's not to say that it makes perfect sense to have any of these races being all buddy-buddy with any of the others. But the rules cover that, with degrees of Allied co-operation, and a situation can always be invented to explain why, say, Eldar and Dark Eldar forces are fighting alongside each other, and no doubt resenting every second of it. Indeed, rather than gut the fluff, the Allied system offers players an unprecedented opportunity to craft their own, extremely particular little corner of the galaxy at war. To return to my own Allies plans, for instance; while the units will come from the Blood Angels codex, I have no intention of fielding actual Blood Angels models. The Librarian and Assault Marines will be experimental battlesuits, built out of converted XV25 models, and the Landspeeder will be a next-generation Piranha, based on wrecked Landspeeders the Empire has recovered in their fight against the Space Marines. It's a perfectly fluffy explanation for everything, and more than that, it allows me to customize the fluff of my own, particular cadre in ways I simply could not have done, before.
Will there be some players who attempt to power-game the Allies system? Of course, just as there were people who jumped to the most bandwagon-y of armies at the drop of a hat. Will there be armies of Eldar and Necrons fighting alongside each other, with no attempt to explain why? Sure, but how many players out there actually have backgrounds for their armies currently? The Allies system at worst maintains the status quo among most players, and at best, allows some players to field truly personalized armies, both in terms of the units on the field and the explanation behind their presence. It's fine to dislike it on a personal level, of course, no-one should be expected to have to like every aspect of every unit and every bit of background fluff. I myself don't field Kroot, not because I think they're bad, but because I simply can't stand the models. But that doesn't mean I think the Kroot's place in the Tau Empire is 'wrong', any more than I think the options opened up by the new Allies system are 'wrong'.
It is what it is, moderately balanced, moderately fluffy. Could anyone honestly say they expect more from Games Workshop?