I had the same type of questions before I started to train in full contact for awhile. I asked a teacher I had, a clinical neuropsychologist about it. He said that getting hit at all, even lightly over a long period of time can cause damage. Of course, I wasn't getting whacked in the head with a bat, so I wasn't too worried about it. I think that getting hit in the head every once in a while won't cause too much damage, at least I hope not, haha.