As an aside, if the big concern with magic classes is that many enemies will be popping up with 300+ mdef... then perhaps the solution isn't to give people mdef bypass, but to just nerf the mdef of the mobs? Mdef works much steeper early on than phys def anyway, and even 100 mdef is a lot.
That's going off the bio monsters, anyway. Unless they're meant to be "idiotically difficult" to the point that you do a single turn in over a span of days instead of an hour.