Cha depends on the character concept.
I have one player that serves as the "face" of the party; Cha 18.
I have one player that plays a rouge that doesn't want to stick out; Cha 10.
I have a player that plays a melee-focused cleric; Cha 6.
I have a player that plays a combat-focused wizard; Cha 6.
I have a player that plays a melee-focused fighter; Cha 10, but only because we pressured him to have a high Cha. He had Cha 4 in one version.
The thing is, in terms of party needs only one character really needs to have high Cha and social skills. Just like only one person needs to have a high Search and Disable Device and some rogue levels. Characters can afford to have low Cha and have others make Cha checks for them; they cannot afford to have low Dex and have someome else have a high AC, or so on.
This is why Cha is a dumb stat, for some builds and games.
Personally, I use Diplomacy checks a lot but never Cha checks. Characters with bad Cha certainly will suffer as a result of low Diplomacy check results. (I don't really follow the RAW or any other rules, I just go with the "feel" or some opposed check when assigning DC.)
Edit: Come to think of it, NPCs also consider Cha itself in my games, for initial gut feelings, and it even has a disproportionate effect: a Cha 4 person would be treated very badly, despite having only -3 to his roll by the RAW.