I think a good interaction system could suggest some bonuses, rather like combat bonuses, that could encourage more role playing and varied tactics. But I'd hesitate to really design too much structure. Whether a PC has a higher ground advantage in combat is fairly objective. Whether a PC is debating the NPC from a superior position (and should get a similar bonus) is subjective, but I think those are the kinds of bonuses that really should be considered. Does the player make a good argument for the PC, at least as far as the NPC is concerned? If so, offer the diplomacy version of combat advantage. That would allow the skilled player to adjust the odds in his or her favor.
All this is, to me, desirable. The player can decide to incorporate in-game knowledge (for example, that the King is a history buff, that they have evidence of Orcish involvement, or bluffing to create an implication of Orcish involvement) to enhance their in-game chances for success. Where I draw the line is granting bonuses because Bob made a really great speech for his 8 CHA, no social skills character, especially when this ultimately results in smooth talking Bob getting equivalent results to Ted's 18 CHA character with 10 ranks in diplomacy simply because Bob is articulate and well spoken, and Ted is not. Ted's character gets no advantages for the fact that Ted is a regular participant and winner of Iron Man competitions while Bob can't make it up the stairs from the basement without a break in the middle to catch his breath. Bob's character should not get an advantage because Bob himself is a god orator, nor should it suffer a disadvantage because Bob is morbidly obese and in horrible physical shape.
It all depends how the DM handles using those stats. If the DM really allows Charisma to be replaced by speech, then effectively you are not using the Charisma skills/checks mechanics in your game, in which case of course investing in it is worthless, or worthy only for a player who is charismatic. If that's how the DM wants Charisma to work, it might be better to even remove the stat from the game!
Agreed. And when Bob's bonuses from his personal skills result in him being able to get results comparable to the character who devotes character resources to persuasiveness, then we may as well remove interaction skills from the game as well.
Just let the player decide what to say in a speech (a mix of player's Int, Wis and Cha), and let their own description grant a small occasional bonus (player's Cha), then make them roll (character's Cha), and you already have a first draft for balancing player's vs character's abilities.
Again, I think the use of in-game resources should grant a bonus. I don't think "Bob is a persuasive speaker" should translate to "Bob's character is a persuasive speaker" any more than an Olympic gymnast should have some advantage on his character's DEX or acrobatics skill.
With this in mind, you can see that physical checks also depend on player's ability because it's the players who decides they want to shoot an arrow or swing from the chandelier, only they don't depend on player's physical abilities, but it's still up to them to make the right decision. IOW you do get good acrobatics, knowledge, search, perception or disable device results with good play and descriptions... it's quite common that a player is rewarded by the DM for guessing exactly where to search for traps or where to climb, rather than generically search or climb, and while this is not about "acting in character" it does require both thinking and talking skills from the player.
Here again, wouldn't an experienced, well trained expert be better able to assess the best places to climb, and the most likely places to search for traps, just as a skilled swordsman is better able to assess the best time to strike to bypass his opponent's defenses? If we have a paramedic in our group, should he get bonuses to the Healing skill for being able to specify exactly how one would stabilize a burn victim after the Red Dragon's attack? If so, why should a player with a degree in chemistry not be able to have his character make drugs, poisons, etc. using his own skills?
The player and the character are, and should be, separate. The player's skill at directing the abilities of his character will, and should, influence in-game success. The player's ability to emulate a character's skills should not.
BTW I don't endorse the idea that players should purposefully do stupid things if their PC is low-Int or low-Wis. I prefer my players to play their best when making decisions, and if they have some good ideas they think their PC could have never have, they can resort to tell us OoC and I certainly won't shut them up!
When your "low WIS, impetuous, impulsive" character routinely takes a reasoned, methodical approach and patiently approaches problems in the best possible tactical manner, I'm not seeing a low WIS, impetuous, impulsive character in play. Instead, I am seeing a playing piece in a boardgame, whose decisions are ruled entirely by the player's interpretation of the best possible tactical choices. To me, at least, that is poor role playing. The "Game" has overridden the "Role Playing" aspect. If the only way I know your character has a sub-par CHA and no social skills is because you read it to me from your character sheet, I don't believe your character is well played. Quite the reverse, if the impression from watching game play is that your character is a persuasive diplomat then I consider your role playing to be poor to nonexistent.