Using "not-evil-dark to help defend people" has already been done with, well, DRK. The differentiating cast times between moving and not-moving seems like it would be a pain to implement, as well - what if you start moving during a cast? What if you stop moving while casting? "Very slowly walk" might also not be fast enough to actually get out of the way for AoEs, which is why you'd want to move in the first place.