Thanks to holy paladin we know that holy healing spells can be cast in full plate armor with no D&D-like change for failing for spells. (In D&D, your spells are very likely to fail if you're wearing armor)
So, you're a healing priest who's going to fight in... lets say Warsong Gulch. Why the hell would you go there with robes? Why not armor? It's a battle! What is the logic behind this?