From a training perspective the danger of hitting each other in the back of the head repeatedly is getting a scrambled boxer's brain. Each blow will kill a whole lot of neurons. One or two times doesn't matter as we are losing brain cells all the time but repeated blows will cause slow degenerative damage.
From self-defense point of view I can't thnik that this blow would cause any more damage than a blow to the jaw (it's just that we all have "glass occiputs"). BUT if you knock out anyone with ANY strike you have to make sure their airway is open (ie recovery position) or they may suffocate and die.