People keep repeating that about 40k xenos. The fact of the matter, though, is that all xeno races were invented by humans, and all info we have on them was written by humans. And that makes them inherently understandable, because the human mind is incapable of imagining something it cannot comprehend.
It's the "official GW line". Xenos are too alien to RP or bother very much with. It's also just another of the many things that makes the setting so attractive to people like myself, and the desire to keep things that way probably makes people like myself prone to repeating it.
But you are of course right. However, the same goes for intelligent swords in D&D, or Nurglings, possessed tanks & whatnot in 40K. My point here being that I very strongly suspect you too draw the line somewhere.
Frankly, I actually find the idea of 40k xenos being hard to play particularly preposterous, because what passes for human in 40k already operates on a pretty alien mindset from our contemporary perspective. If you can get into the mind of a superstitious bigot who accepts every command from his superiors as literal word of god enough to play him, you'll be fine playing a stuck up, pointy-eared guy who believes in destiny and is willing to commit any atrocity for the survival of his race.
Actually, I think that's a pretty good argument against xenos as PC material. My own group's first campaign ended up in large part being about figuring out how to be a human being in the setting, precisely because they're so alien to us. I'm most definitely not seeing the real aliens being easier to relate to, at least not with our kind of play style (but then, we're a bunch of EMO's
).
As I said earlier, though, I don't really have a problem with playable xenos as long as the assumptions, content and tone is kept separate from the other game books. Because I don't want what you want. I anti-want it.
It shouldn't be super-tricksy to do, though, assuming GW gets out of the way. Contrary to what Togath says (no slight intended), most of the race fluff is quite well-developed. A whole lot of the early material was written, intentionally or not, in a way that made it very RPG friendly. And besides that, the wargame has continually introduced new fluff elements with every update (and there's been a few by now), and there's the very fluff-heavy "enthusiast games". There might not be a 20-book game line's worth of fluff for each and every xenos race, but for the ones that have miniature armies there's easily both enough and the right kind of fluff for a stand alone RPG core with a couple of supplements for each of them.