I have, and more than once. One of the guys I gamed with in college was extremely withdrawn, and his speaking voice was almost a whisper* and often quite slow & deliberate. Having him RP a social situation would have required more than the short sentences he would utter quickly in combat, like "I shoot him" and thus would have been extremely drawn out. Were it not for that game's social skills, the GM would have been exhausted by session's end. And despite all this, he always was pleasant to game with and had cool PCs who did what you'd expect those PCs to do. IOW, he grasped the process and contributed to the game for all.
Without those systemic social skills, though....
How did this work out in play? Did he play supposedly loquacious PCs? Did he paraphrase what his PC was saying, and then rolled a skill check?
IME, If the player gives enough information in the paraphrase that the GM can understand what the PC is saying, this can be workable, though I don't find it much fun.
My normal preference though would be that if a player doesn't want to talk, they play a laconic PC. Conversely, in [MENTION=9037]Elf Witch[/MENTION]'s case of her getting tongue-tied sometimes, I can't see that being a problem in my games; stuff could come out a bit wrong but exact words are really not nearly as important as a lot of other factors in getting a good reaction - things like attitude & demeanour matter far more.