well certainly a mental-stat bonus is going to be useful to less classes, as far as their actuall class abilities go.
A Str bonus is a big thing for a Fighter, Paladin, Ranger, Monk, Rogue, regardless of specfic focus. Likewise with Dex, and Con especially is very useful to all classes.
But take Int for example...skill stuff aside, as far as actual class abilities the only class it benefits is Wizard. And it gives you more skill points, which is very nice but from a combat perspective, mostly not that important.
Wisdom is only super big for Clerics and Druids, and also of slightly less importance to rangers, druids and monks.
Cha only effects the class abilities of sorcerers, bards and paladins.