I don't recall getting Mage more than once ever, either. Once you have it, you realize how much of a waste the pursuit was. In FFIIUS, the only worthwhile monster-drop summon is bomb (but it is very worthwhile). My impression is that, given the frequency of the baddies that drop it, the time at which they begin appearing, and the effect of the spell, the cockatrice summon might be worth it, too. But then again, unless you're hella lucky, you're probably leveling shadow party Rydia out of the need for it. Those spells (except perhaps Bomb) should probably have been set to a higher frequency.