There are a few dialogue stat checks in the game, 15 cha is enough for the charisma based options, there's also a beneficial option if you have 14+ int or wis.

A fighter with 3int,wis,cha has no disadvantages compared to one with 18 in those stats.

Druids, paladins and bards don't benefit from higher charisma than the minimum. You only benefit from having one high charisma character for dialogues and shop prices.
If you have a bard the 15 cha minimum is perfect for dialogue and you can cast the "friends" spell to raise it by 5 to 20 which grants optimal shop prices. There are also some bard specific dialogue options in the game.

Druids need high dex for good AC and a bonus to ranged attacks, con for bonus hitpoints and strength for carrying capacity and doing physical damage with melee weapons and slings. Extra spells for high wis are also nice, int and cha are useless.

Paladins need str, dex and con. Int, wis and cha are useless for them.

Bards benefit the most from 18 dex, 16 con gives them extra hitpoints, 18 int is essential for scribing scrolls. Charisma can be kept at the minimum. Wisdom is only useful for lore, since bards gain 10 lore per level even the penalty for 3 wis won't hurt any more after the middle of chapter 1. Strength is useful for carrying capacity.