I can see that if a spell caster in general casts an offensive spell at anybody, that person, if wanting to defend/resist the spell in question should use a mental stat. As I have seen from other posts, it isn't fair to let the warriors resist magic with Strength. There is no need for it, unless there is a secondary effect that would push the individual, then maybe he can resist the pushing effect.
On a personal note to the conversation, I can see all spells with the exception of mentally effecting spells, like charms and such, would use Intelligence as their defense for a reduced damage or even half damage. With the rest of the charms and illusions, I can see the use of Wisdom to determine whether or not the person is effected. Granted, spell casters in general will be better suited to resist spells and not so for the more physical attacks of martial classes or the more physical monsters. But in the same retrospect, martial classes with weaker mental scores would not be that great at defending themselves from magical attacks in general.
Back on one post from "Fighter" I believe, if a black mage was to bust you with a fire attack, you do have means of a counter attack, run up and bust him in the chops with said weapon in hand. If he is in a position that melee attacks are not going to work, i.e.: on higher elevation, use ranged weapons, but if that is out of the question, then you are screwed until something changes the given scenario.
Ok, enough ranting. My 2 cents are put forth. Have a nice day.