Spell Hit vary helpful for a prot pally more so than expertise rating. Remember you don't build aggro with white damage, its all spell damage. A good prot pally may not even need to hit a mob inorder to keep aggro on it. I don't how many times I've been resisted by a mob or bosss that resulted in loosing aggro. The higher raid instances it is very important for DPS to max spell hit or melee hit rating inorder to be effective.