That still doesn't answer the more important question: why, as DM, should you make a big deal out it? Let the mechanical penalties fall where they may and leave the actual play and characterization to the player.
Because, as I said, in the first place, it's very easy to evade most mechanical penalties. A decent power gamer is going to select disadvantages that provide no mechanical penalty that they care about at all. Besides which, imposing mechanical penalties that effect player choice regularly gets to be very irritating for both the DM and the player. If the player takes the initiative to act out his disadvantage (this is more applicable to systems other than D&D, say GURPS) there is little need to go to the blunt instrument of mechanical enforcement.
And in the second case, as I previously indicated, it is because this sort of behavior tends to irritate other players as well. Often times they see it - rightly IMO - as an attempt to hog the greater share of the spotlight and glory for themselves, while doing nothing to entertain the other players. It's just not good teamwork to say, "My player is good at everything.", which is at the core of the game what the rules exist to prevent.
As to why I care in the third place, the answer is very simple - if the PC's don't roleplay well, the game becomes rather unentertaining. A gamer who dumps stats all his mental and social abilities expecting to rely on his ability, knowledge, and cunning as a player to be cunning, charming, persuasive is in my experience not only not at all interested in roleplaying but very likely to begin to argue in a metagame fashion about how my ruling that the player's charming, persuasive, reasonable statements ought not to have provoked a negative reaction from the NPC 'merely' because a dice throw indicated a fumbled diplomacy check from his CHR 4 oaf. I care because the game just is more fun to play when the player takes the responcibility for policing themselves rather than trying to see how far they can push, manipulate, or bend the system (and hense me).
A 6 INT PC will make a terrible wizard.
So what? No one likely to pull this kind of stunt will dump stat INT when playing a wizard.
So what if he or she frequently comes up with smart plans? Perhaps the character is possessed of a certain low, dumb cunning --why they must be, they keep coming up with crackerjack plans!
Again, someone who is playing a character that communicates 'low, dumb cunning' is probably entertaining, and probably would have never dump stated in the first place and instead is playing the 11 INT as meaning 'not that intellectual but has a low dumb cunning'. Someone who takes 6 INT as a dump stat and then plays the character as a mastermind probably isn't ever in character and hense never entertains and probably has this whole adversarial PC vs. the DM thing going.
As DM I get better results helping realize and rationalize my player's characters than constantly trying to evaluate if they're playing them correctly.
Assuming that the player's have characters in the first place. It's the blatant lack of characterization and of any thought actually given to character (as opposed to 'system mastery') that is the heart of my problem with this.
