well, the mages would be given a bit more chance for good defense, but so will knights.
And about what you said about spells, you got a point there, mayb there should be a penalty for that like 1,5 or even double casting time with duo shields.
And i dont know anything about the AI here, i just got this idea from a other game with those circumstances, so you guys know how i experienced that feature, and that i could turn out better or worse with other AI.