The higher the enchantment on Magicbane, the less
likely it is that you'll get one of the special attacks out of it. It is Magicbane
, after all, and it doesn't like to be enchanted.
+2 is the optimum enchantment; any higher than that and the useful side effects go down rapidly. Being an athame, a magical tool, it can engrave the magic word in one turn and unless it's cursed, do it without being dulled like any other weapon would be.
Stormbringer is less than useful to Wizards, the most likely class to be given Magicbane, as they are only able to get to basic skill with it, where they can get to expert with the athame; using it also raises your dagger skill, thus making you more deadly with thrown daggers. Not to mention if you move in the wrong direction it attacks things on its own, so you can lose pets or worse, hard-won intrinsics like telepathy.
And any weapon that can kill a just-spawned Pit Fiend in three hits is for me.