Dausuul? Why? Because it's much, much harder to provide guidelines for functional play with evil PCs (and it is!), and most play is heroic, they've went with "heroic party, no evil PCs" as the standard ruleset.
Of course you can break this and still have functional play; it just requires knowing what you're doing and not being a jackass. But if you do so, you're already playing off-warantee, so it makes sense that when they're presenting "evilish" options for general consumptiom, they provide flavor in the best possible light. After all, they -are- providing options like the half-orc, the reventant, the assassin, the warlock, and genasi, and the minotaur (and, of course, the drow), so it's not like they're not giving out mechanical crunch that can be used to dark ends.
I'm not seeing where genasi are a "dark" race; I'll concede the rest. That said, I think the "functional play" argument is a dangerous road to go down, because it leads designers to don intellectual blinders. Good ideas get rejected out of hand because it's something you "can't do."
Look at what happened with magic items. There was a while when WotC was claiming they "couldn't do" powerful and flavorful magic items because they would result in non-functional play. Then somebody took off his or her blinders long enough to think up the item rarity system--and now we're going to get powerful and flavorful magic items.
I think the same thing is worth considering for "evil" races. Taking the yanki/zerai example: People want to play githyanki. If there is sufficient demand (which admittedly is an open question), I think it's worth searching for a way to meet that demand, and it shouldn't be a deal-breaker if you can't come up with a way to cram githyanki into a "heroic race" mold. Think about other ways that githyanki could function as PCs. The typical PC in a "kill monsters and take their stuff" game is a violent sociopath anyway.