I think there is a well summed up argument for wearing a helmet, I have knocked myself out a few times wearing a helmet, what would I have happened had I not worn a helmet? I have a different helmet for each dicipline I ride, over the top maybe but it's important to be safe, especially having seen the effects of a crash my friend had, he was an xc rider, got knocked off his bike landed on his face. I went to go see him in hospital and I did not recognise him, his face was gone, his parents had to take a picture so that the doctors could reconstruct his face! The only thing that saved him was a helmet! Trials is a calculated risk, minimise the risk further with a helmet!