GameBanshee Forums

Charisma deals with how npcs react to the speaker. In some areas a high charisma would be the only way to get a reward(in gold). And there may be a few incidents where you can get attacked unless you have a high char.
I'm sure you can get an npc with a high char.

High Charisma makes shop prices lower, enables beneficial dialogue options and can let you wear some items.

For instance, your charisma must be 13?? to get the boots of the north from Jared when rescuing him from the mountain bear.

Spoliers,

Not a definitive list, but a Char of 18 gets you: A +1 dagger from the bloke in the barracks who wants crossbow bolts, 6 potions of antidote from the Gnome on level 2 of Friendly Arms who want Spiders in Bergost killed, a cash tip (5gps) from the Pantaloon man if you take them for cleaning, an offer of 400gps instead of 300gps (200 gps if Charisma is >7) from Silke. So a high Charasima in BG1 is Much more important than in BG2 where it effects very little and a certain ring makes it all better on day 1. - Curdis

Still, playing through the game with Charisma 3 would not be that much different from Charisma 18. Same goes for Int and Wisdom. At least, the difference would be a lot bigger for Strength, Dex or Con.