Depends, too, on how you use it... with the way blocks work (ie, you can block anything- a shield spell is simply a block vs damage centered on yourself)... instead of setting up a block vs incoming damage on yourself, you could aggressively set one up on a given enemy. They'd get an initial chance to dodge your spell, but instead of it doing damage, they find themselves in a bubble of force that hinders them attacking anyone.
Given enough power, it's a quick, effective, non-lethal way to neutralize a single enemy for a round (assuming they can't overcome it), then extend it the following round if need be.