Shield and casting should work together not be opposed.

[Lambdafish]

Agreed. It would make sense given both white mages and black mages CAN equip shields that there be some reason to them even having it?

Its ironic that white mages and thaums both can wear shields...but they are useless while casting.

Also I keep wondering if they'll ever do the shield thing with BLMs or WHMs again. It feels like it was some artifact of leveling that just gets discarded.

BLM/WHM shields are a side effect of 1.0. The devs never liked it and, like classes, would happily remove the functionality entirely in the game. Its current state is the best they can do. They will never come back and shouldn't be used as an argument against this; regardless though, I am in full support of the OPs suggestion.