I'm against it, mainly because I don't think I've ever played with particularly good roleplayers.
That is, most "roleplaying" that my players do falls under one of two headings:
1.) They basically play themselves, regardless of what's on the character sheet
2.) They pick a few traits they see as defining the character, and then embody those traits almost to the point of parody. So the nerdy, arrogant wizard becomes essentially a caricature of an arrogant nerd--he's always being a know-it-all and sneering at the "plebes" around him, or he's being a nebbishy wimp. He feels like the "brain" in a 5 man band from a Saturday morning Cartoon. Or that dumb, brutish half-orc barbarian is an absolute idiot in nearly all situations, and all he wants to do is hit things. If they do mix it up beyond this, its nearly always with a "humorous" incongruity, like the half-orc barbarian loves fluffy bunnies or something.
This is understandable. I even do it myself, for the simply reason that most roleplayers aren't master thespians (god knows I'm not), and even if they were, bringing a nuanced character to life in a way that feels "real" is really, really hard (look at how many times Hollywood fails at it, and they've got not just professional actors, but teams of scriptwriters). The bar for roleplaying falls, in my experience, significantly below the bar one would set for actual actors bringing characters to life in a tv show, movie or play.
And thats fine.
Where I have a problem is when the "defining trait" about the character becomes a gender or a race, because IME, most players simply aren't good enough to build much additional nuance into their characters beyond a few "defining traits".
So I've had players who, in modern White Wolf games, stated that they were "gonna play a black guy." And then that character became a caricature of "blackness" in the same way that the arrogant nerd wizard became a caricature of arrogant nerddom. Except,caricatures of angry nerds can be funny, or at least, not-offensive. Caricatures of "blackness" as the average white nerd sees it...really aren't. That black fighter calling people "foo" or "homie" (or worse "my brother") and saying things like "sho nuff" just makes me really, really uncomfortable and takes me out of the game (in case you can't tell, this actually happened in one of my games).
A similar thing tends to happen when my male players play women. Either they write "female" somewhere on the character sheet, and then proceed to basically ignore it and play the character the same way they play all their male characters. Or they decide to "roleplay a woman", and what emerges is a horribly twisted caricature of what some geek apparently thinks women act like (typically, ditzy, shallow and/or slutty. with bonus points for "comically" high-pitched voice).
I've never ONCE seen a male roleplayer play a female character in such a way that she felt noticably "female", without becoming a caricature, and while feeling like a fully fleshed out character in her own right.
Given how rare male writers who can effectively write women, let alone male actors who can effectively play them, that's to be expected. It just means I'd prefer they don't attempt it in my games.
If others have had different experiences, then I'm jealous, because apparently they're playing with much better roleplayers (that is, actors) than I am.