I can't imagine moving around and doing anything physical with a shield around you, but all of those times that someone was standing somewhere up high with a good view of the battle so they could throw fireballs? A shield would protect them from the lucky arrow shot at least!
That does make me wonder, is there are reason they couldn't make a shield and tie it off so it wasn't constantly taxing them? At least something simple like a cylinder around your torso or head, maybe.