The head doesn't drop aggro from phase 3 to 4 so all the warrior has to do is taunt the head and then the phase 3 ranged tank drops aggro. That way you can have 1 healer focus the tank, 1 healer on raid, and the 3rd helping out wherever needed. You do that instead of 1 healer on warlock, 1 on main tank, 1 on raid.