I've always considered it a lot more easier to hire voice actors without much experience. That way you can whip and kick them into doing what line you exactly want to hear without ego's flying off the room - plus it's cheaper.
Charlie Murphy could've been hilarious as CJ... IF he had the lines(script) which would allow him to be so. If not, then it wouldn't be any different than hiring some random va. On that note, who the hell is Young Maylay?
As a matter a fact, would people even be the least bit more interested in the game simply because the va's are "famous"? Then again..... *looks at "Shark Tale" voice credits...
I definitely see your point about it being more immersive.
Having a voiceless main character simply enables the player to imagine how or what his/her character would actually sound or say in the various presented situations. So you don't have to worry about "you" sounding like an idiot (ie Fable, Zelda, GTA3 blah blah).
That said, if the vo's are done well then that problem shouldn't even arise.