What if you're playing a game with a social mechanic task resolution (3e and 4e both have one). The "charismatic" player has an 8 charisma and did not train any social skills. Yet he always seems to dominate every social interaction (the player always seems to know just what to say in any given situation), are you going to reward that?
Other than good faith (which is important, but we are talking hypotheticals here), what's to prevent the above player from completely dumping his CHA (let's assume nothing else in his build requires it) and not bothering to pump his social skills, Thereby gaining a significant mechanical advantage?
I think these two issues are related.
First, I have to say basically all my GMing experience has been with "good faith" players. Basically, I don't have players who try to "cheat" by building PCs who are mechancially weak at task X, and then trying to end-run around the action resolution mechanics for X.
But there are ways of not cheating, and playing with good faith, which don't require the charismatic or invested player, who has a PC with a low CHA/social skills, to check his/her charisma and energy at the door.
In my own case, this is mostly handled by making the situation more complex than simply "does the PC give a sincere and convincing speech".
In my sesssion yesterday, for example, the PCs had to break the news to the Baron of the city where they are staying that his niece, who was missing and whom they had been searching for, was in fact a necromancer whom they had caught red handed dealing with the undead. They brought her back to him as a prisoner. She was then sent off to take a bath, but told not to leave the building, while the PCs finished negotiating with the baron - but being a GM ever-ready to dissapoint my players, I decided that she sucked the life out of her ladies-in-waiting, teleported out the window, killed two more guards and tried to flee across the river. She ended up being killed by the drow sorcerer getting a lucky shot with his longbow. The PCs then had to report back to the baron that his niece was dead, killed in the attempt to apprehend her after she committed multiple murders (besides the ladies and the guards, she also sucked the life out of the boat pilot in an attempt to survive the PCs' attacks).
In this scenario, the general pattern of the social interactions with the baron was determined by player ability, not PC stats - the charismatic players get to set the tone for the conversations with the baron, responses to his responses, etc. But the way the baron dealt with the news that was being given to him depended upon the die rolls - the PCs with good Diplomacy were able to break news to him in a way that was gentle (relative to the circumstances) and didn't crush him, whereas those with poor skills were obviously overburdening him with bad news. And when this became apparent to the players, they adapted, letting the more comforting PCs take over the scene while the more dominant, but less skilled, pulled their PCs back a bit, and threw their energy into other things.
I also find that by making even charismatic players make social checks - so they get to play a big role in framing the fiction, but still have to roll dice to see how it pans out - you overcome the "problem" of players only ever rolling for their good skills. The dwarf fighter in my game rolls plenty of social skill checks, even though his bonus is only +7 (14th level, 11 CHA). And when he fails - as he often does - it doesn't mean "You make a fool of yourself as you try to charm but spit in their faces instead". Rather, he doesn't achieve his intent - in the above example, the Baron takes the news badly rather than accepting it, for example. (This is my understanding of Burning Wheel's "intent and task" approach to action resolution - set a skill appropriate to the task, but resolve failure having principal regard to the intent.)
