I think it’s a more relevant problem for
@Lanefan than it is for most. He’s pretty open about the fact that he and his players take a very no-holds-barred approach. When the expectation is that the players can and should use every advantage they can get, it makes more sense to explicitly disallow dumping a stat and then playing the character as if they had a high score in that stat (assuming that the group
wants ability scores to have an affect on characterization).
I have a real hard time with the mental ability scores. Like, sure, it makes sense that you shouldn't roleplay your character being articulate with a low Charisma, or a capable tactician with low Intelligence- and yet, there's no clear threshold of exactly
how charismatic or intelligent or wise one needs to be to roleplay their character in a certain fashion.
Worse yet, pop culture is full of characters who are woefully handicapped in some areas, but then have surprising competence in others- a classic example is the shonen protagonist who is totally lacking in book smarts and may even come off as an idiot, but then becomes hyper-competent with regards to strategy in combat.
Given that the 5e system doesn't give some ability scores much to do outside of skill checks, there's no real way to police this sort of thing. Saying "sorry, you can't be Hannibal Barca, you only have Intelligence 12" is completely arbitrary- how much Int does one need to invest to claim to be a mighty general?
Worse, even if one did make an 18 intelligence warrior, the system doesn't give intelligence any real way to benefit a character in combat; they can be no more intelligent or savvy than their player is.
Even when we do have rules for the use of ability checks, like with social rolls, there's still the classic paradox of the charming, articulate player and the introverted shy player- you still get DM's who insist that social rolls be roleplayed (because, I mean, it is a roleplaying game), and are leery of "allowing" a low energy Charisma roll to succeed on the merits.
In short, it's easy to say that someone is playing above ability of their character, and demand higher ability scores, but in some areas, players are not given much leeway if their characters are more capable than they themselves are.