What are the practical implications of including the "mental abilities" of a character during play? Is it preferable aesthetically for a player to "play the stats" of the character? Or should the player's choices be driven by the practical circumstances of the game and the guided by the player's ability as much as, or more than, what's written on the character sheet?
I've been backwards and forwards on this a couple of times. At one point, I even rewrote 2nd Edition to remove the 'mental' stats entirely, following one too many instances of players min-maxing and then ignoring them. However, on balance, I feel the game loses more than it gains from this move.
These days, my view is that player's should 'play their stats', but that I won't enforce it. However, I don't let players either handwave or role-play their way around the limitations they have built* into their characters. If the player knows something pertinent, he doesn't get to bring it into the game without the appropriate Knowledge check. The player may be as eloquent or intimidating as he likes, but he still has to make the roll, or his Cha 5 Barbarian won't be intimidating those goblins any time soon.
(I have a player who will ask at the start of every campaign, without fail, whether I'm using his pet House Rule that you can use Str to intimidate. The answer is always 'no'. Presumably, your Cha 5 Barbarian has a squeaky voice, or comes off as oafish, or something. Either way, you don't get to Intimidate without either the score or the skill ranks to back it up.)
This, actually, is why I'm not a fan of giving "role-playing" bonuses to Diplomacy and Intimidate checks. If the player is very slick and eloquent, but the character is not, then giving the bonus allows the player to re-assign those skill points somewhere else for no loss. Conversely, if the player has little social grace to speak of, he shouldn't be automatically be banned from playing a dashing Bard, any more than the inability to cast spells should prevent him from playing a Wizard.
For example, should a fighter with INT 5 be played as "dumb?" Or should the player play the character to the best of the player's ability with only minimal or no regard to the ability score?
For the most part, if the character sheet says the character has low intelligence, low wisdom or low charisma, the player should portray the character as such. The DM may need to occasionally remind the player of this, but shouldn't enforce it. (That said, if a player has a history of min-maxing his character, and then routinely ignoring the low stats, there may be scope for the DM requiring that player to assign a certain minimum value to those stats for future characters.)
However, there is one big exception I make to my "play the numbers" guideline - when the group is engaged in collaborative problem solving, I would argue that all players should be fully engaged. The notion here is that while Genius Bob's character may be an Int 5 barbarian, Nice-but-Dim Tim's character is an Int 18+ Wizard. So, to "make up the gap" between the two, I feel it is okay for Bob and Tim to work together. (It's probably best to assume that Tim's
character comes up with the answer, even if it was actually Bob that put it all together.)
* It should be noted: I always use point-buy these days. So, if a character has a 'dump-stat' assigned, this is because the player
chose it. If the character doesn't have ranks in a given skill, that's because the player chose to spend them elsewhere. Either way, I don't see a problem with the player having to deal with the consequences of these choices - they do, after all, get the benefits of higher scores in other areas.