Reader's beware: not a happy post :P!
It's Cinematic. Forging a narrative. It's the 6th edition catch phrases and of course we love applying them to everything. Want to know what it's best applied to? Allies. The universe we all love and enjoy is bloody vast. We've all read lots of fluff background, books, stories, etc. and we've seen lots of crazy combinations. Off the top of my head I can remember Eldar & Imperial Guard allying, Space Marines & Tau, Blood Angels & Necrons, etc. I can remember Grey Knights allying with anything. And then killing them once their task is done.
It's fucking Cinematic.
And you know what? Allies allows you to do this. Things might not make sense on the face of things. Imperial armies and Xenos don't generally get along and although it's very possible to see Imperial Guard allying with anyone and anything (humans suck - they are so corruptable and easy to seduce with boobies), the entire Allies matrix doesn't always make sense. Guess what? It doesn't have to.
It's bloody Cinematic.
The fluff has a well-documented history of alliances forged under necessity. Hey mortal enemy! The Tyranids are coming and going to EAT us. Want to stop our pathetic mortal battle and like...live? Sure says the mortal enemy and then both of them plot how to screw the other royally once they both live but band together to blast those crazy bugs back to space! Because living and stabbing your mortal enemy in the back rocks. Haven't I seen a movie like this? Or read a book about it?
It's damn well Cinematic.
Dying isn't. Dying ends the Cinematic and that's clearly why Tyranids cannot have allies. They are un-Cinematic. I've figured Games Workshop out. The most Cinematic stuff is heroic individuals like Space Marines kicking arse. They get lots of Allies to therefore be the most Cinematic dudes possible. It's harder to be Cinematic without showing off to your best buds or worstest enemies. You're not being a show-off - you're just one of those sad guys who thinks their heoric stand will go down in history and people will toast them with ale later in the millenia. But if you've got all these randoms watching you?
It's stupendeously Cinematic!
And Imperial Guard can ally with lots of people because...boobies. Didn't I explain this before?
It's erotically Cinematic.
What's the point of this you ask? If you don't like fluffy play, that's okay. You've got Allies to make a wealth of really good & balanced lists which just wasn't possible before in 5th over so many books. If you do like fluffy play, that's okay. YOU'VE GOT A CINEMATIC MATRIX. And that movie was Cinematic A+. Some of them might not make sense to you but it might make sense to someone. Someone might find it fluffy & interesting or think of a good background story for such a combination of armies & units. Good for them. They're helping Forge the Narrative and making your game Cinematic. Don't cry and whine about the system, particularly if your issue is someone using it to make their army better. Because...
That's not Cinematic and that's poor form laddies.