I feel that the MW's mastery is very powerful. The issue is that people are too dumb to actually utilize it. I can't tell you how many times people ask me to heal them after a fight while a healing sphere is spinning right next to them. It also bugs me when my party leaves a battle and there's healing spheres all over the place.
I don't know what the solution is really. I think the mastery is actually very cool and fitting for the spec. But it's just amazing how ignorant people are to our abilities.