Preventing the damage in the first place will always be stronger than healing it afterwards. Shield stacking is way too powerful. Removing Deployment Tactics doesn't balance it. It's funny that you mention WoW because I'm pretty sure you can't have multiple Disc priest shields up at once and they're really the only Shield-focused healer in the game. (Though WoW is a very different game, they don't just have 2 flavors of healers.) Your change would completely lock out WHM and Diurnal AST.