and someone who uses charisma for a dump stat but tries to play the smooth operator is just as likely going to get a penalty from me as DM for bad RPing.
I actually have more of a problem with the CHA 7 PC, who to my mind might be a bit reserved and withdrawn, being played as an obnoxious jerk who annoys every NPC he meets. At least in a socially oriented campaign such as a city-based one, I'd prefer it if players didn't intentionally disadvantage themselves beyond the requirements of the dice.
When it comes to very low CHA, I recall a CHA 3 half-orc assassin PC who disguised as a beggar and was played so repulsively, people he met would ostentatiously ignore him, letting him shank them in the back. I thought that was brilliant Gamist use of a low stat. This was possible in 1e as Disguise skill wasn't dependent on CHA. I've had a lot of trouble with 3e's link between stats and skills - low WIS Rogues who can't spot things as well as the Cleric, low CHA Rogues who can't disguise themselves, etc. I find 1e's discrete sub-systems allow for a lot more flexibility in the type of characters played.