Quote Originally Posted by Aureact
Although shadowform has become a lot more crucial to the shadow priest, which means that dropping it for healing could be absolutely devastating in terms of winning an arena match. Sure you could put it up again, but that's 10% of your mana pool gone.
The manacost usually ain`t the devastating part but the fact that you go into defensive mode and offensive comps can`t do that too well.

If someone ain`t dead when you drop the form you are usually pretty fucked that point(this goes only for arenas and bg`s)