6th edition is a little over a year old now and I've had some time to consider some of the things I wasn't fond of when the new rules dropped. After a year, several new codexes and attending a GT I've come to the conclusion that...yeah Allies still suck.
OK, first off let me acknowledge this: Allies was added to the game just to sell models. You know it and I know it. GW is a business and this was a decision to encourage more business. I'm fine with that because if they don't make money, they stop making good games (and the Hobbit which still sucks)
I really don't like allies in general an I think it leads to lazy list building. One of the key strategic tenants of gaming in general is playing to your strengths while minimizing exposure to your weaknesses. If your army is great at shooting but can get sissy slapped into oblivion by something half-way decent in close combat, you used to have to plan for that and keep your enemy at arm's reach. If they get to you, then you're in trouble. My job is to find those weaknesses of yours and exploit them while protecting your own. Allies gives you the easy button...take an ally to shore up your weak spots and you don't have any.
An individual codex is internally balanced (in theory...screamer star notwithstanding) but adding allies into that just throws what little play-testing goes into balancing the game straight out the window. Ever face a Tau-Eldar combo or an IG blob with DA so the whole damn thing has ATSKNF and an invul save?
So allies suck. They are too easily broken and take away from the strategy of list building. Can they be fixed? Well, to a certain extent I think so. One thing I didn't understand is the grid. Using that grid, it was easily possible to have different levels of allies based on which side of the equation your main army was from. What do I mean? Well if your main detachment comes from codex A, you can take an allied detachment from codex B that are battle brothers, but maybe codex B doesn't trust codex A as much so if your primary detachment is from Codex B, them maybe you're just allies of convenience. This also would let Tyranids play with friends as while I see the hive mind not bothering with allies themselves, maybe an IG army can take an allied Tyranid detachment and oh look, there's the Genestealer cult players have only been asking for a return of for the last decade or so.
Oh and with the exception of IG being an ally of chaos and Tau to represent cultists and Gue'Vesa, no Imperial army should be able to ally with any xenos. At all. This is my other big issue with allies. Some of the partnerships make no sense. Hey, you know how every single piece of fluff written in the last few decades says humans are largely focused on eliminating anything out there that's not human? Well don't worry about it because space marines will totally bro out with all those aliens they've been trying to exterminate for millenia even though this edition is all about "forging the narrative" Eldar and Dark Eldar should at best be allies of convenience because you know...they've only been slightly annoyed at each other since before humans learned how to light fires but whatever, right?
Now we're seeing rumors that Fantasy will have allies, too. Hey, you know how your army only has access to certain magic because some may make you overpowered? Wait a while and we'll fix that. After all, when everything is broken, nothing is broken, right?