Yubello's magic is so bad that he really really wants the extra magic growth from Mage.
It's an interesting question whether it's possible for Yubello to proc magic as Curate. Perhaps the game just can't deal with his magic (which would technically be -1), because when he procs magic, he'd have to go from 0 to 0. I'm curious to see you level him all the way to 20 this way. It will be, for Science!