Aside from training magery, it has no purpose (that I know of). Don't construe that as "no reason" though. A -magery weapon can be equipped by a mage to do melee battle at a skill level equal to his adjusted magery score. This means, a 100 real skill mage would wield a -23 mage weapon as if he had 77 points of skill in whatever type weapon it is. Using said weapon, he would also be able to gain magery skill as if his magery level was actually 77, just by using the weapon. No casting necessary, just swing and gain. Now, of course, it can't all be that simple, as a mage generally isn't as tactically proficient as a warrior, so must be a bit more judicious in what he chooses to fight!