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In this article, good ol' Larry makes a lot of the points that I feel strongly about in terms of Allies. On spec, I love the idea - we've all wanted to be able to field our Imperial Fists getting pulled out of the fire at the 11th hour by Deep Striking Blood Angels. That's awesome! It's cinematic, it fits the fluff, it's good army building. All for it!
Or, any of the "Good" (re: not Chaos) armies against Daemons/CSM. Or any combination of armies against the Tyranids, Necrons or Orks. Do some narrative here! Enemy of my enemy is my friend stuff.
BUT, those combinations really only make sense in campaigns where you know who your opponent is. Space Marines/Eldar vs. Space Marines/IG?
I've even seen Imperial Guard modeled as Traitors and Allied with Daemons. That was awesome! Why? Because Traitor Guard are a thing in the fluff, and the guy took the time to model them so it made sense.
Take Eldar supported by Eldar Corsairs, CSM and Daemons are a match made in Hell (in a good way). Heck, I would even take Orks with a small detachment of CSM or Eldar, so long as the Allies kept far out of the Orks' way and some back-story effort was made to say the Orks were being used by the machinations of the Allies.
...There every pretense of fluff is thrown out the window.
I played against a guy who clearly thought "I love my Space Wolves, but you know what they need? Wave Serpents!" He even threw in a Wraithknight, but didn't even bother to bring the model. This was a tournament, and he borrowed a Flying Hive Tyrant from another player to proxy his Wraithknight.
I've seen net lists that just make no sense, and are just a mishmash of all of the most powerful units in the game.
I'm especially annoyed by the incredibly popular Eldar/Dark Eldar Alliance. I mean, really? The Dark Eldar are an undisciplined remnant of what caused the destruction of the entire Eldar race! And they would be just as - if not more - likely to kidnap and torture to death an Eldar hero as they would a member of any other race. There's fluff that the Eldar want to preserve as many of their number as possibly at the expense of any other race, but I do not think that truly extends to Dark Eldar. They exist only to feed Slaanesh a little bit more. Bleck!
I could go on for hours, but let me close with the single best piece of advice any gamer can give another:
Don't be a d*ck. Play for the game (which includes modeling and fluff), make some effort to have your army make sense, make it competitive, but don't toss your units aside in favor of the best from other armies.
I don't ask that everyone be a pro painter, or even necessarily have all of their models perfectly modeled with What-you-see-is-what-you-get (WYSIWYG), I know things change too quickly to always do that, but make some effort please.
That's all I ask for:
Not have the deep desire to slap my opponent in the face for ruining the fun. A gentleman's game, where I can shake hands at the beginning and end of the game with a smile on my face whether I won or not.