SWTOR has a similar system for their jedi/sith casters/healers.

Force is capped at 100 and you have to work with that, of course it would regen at a set rate. Problem is, you'd find yourself OOM very quickly and often if you weren't careful.

Point is, now that they've capped MP, they absolutely have to balance MP costs or healers won't be able to do their jobs. That is my main concern. SE has shows moments of brilliance with balance, and moments of sheer stupidity with balance.