I normally wouldn't care about defend either, but in this case it would give the mage something to do that isn't potentially detrimental. A system that makes it so that everything a character can do will harm him or the party is a stupid system, personal tastes aside.
When I played through I made two of the party members into magic users, so for each battle there was not one but two characters for which I always had to choose between wasting mp and lowering my magic power. twice the stupidity.