I can say from personal experience that RP XP in a group of experienced gamers is very different from when a player is not familiar with the rules.
The first White Wolf game i played, the DM gave out the WW equivalent of XP at the end of each session by asking you "What did you learn". I was pretty unfamiliar with the system and had created a rather weak and ineffective character, and had a hard time contributing alot to the general flow of things, but i tried, i was just unfamiliar with what was effective. So when it came to me, i really never had much to say because A) i really didn't know what the DM was looking for. B) My character hadn't learned all that much on his own, because he was gimped by my inexperience. As a result i got about 1-2 points per session, while others with their tweaked out characters were getting 4-5 points. as a result, their characters could grow, mine did not. It was very discouraging. I eventually got better at it, but i definitely felt put down by being punished for being new.
Roleplaying xp in D&D can do the same thing. A new player will seldom roleplay as well as the 5 year veteran, and thus the rookie will effectively be punished for being new, and his character, and effectively his roleplaying skills, will be stunted. This is discouraging. Also, this encourages spotlight hogging, since the person that does the most talking will argue he did the most roleplaying.
On the other hand, if you have a group thats experienced OR willing to encourage the new players, adding in xp for cleverness and getting into character is fun, as long as everyone has equal access to it. I once played in a game where we got 25xp for every fly bugging the DM we killed. It was silly and fun (and not techincally role playing but oh well).
Its the DM that makes or breaks RP XP, WoTC is wise not to try and encourage it in general, IMO.