i wear a helmet not for the fear of hitting my head (which i haven't done really badly, knock on wood) but because one time on a pow day i was cruising and got bucked off of a shallow log and scorpioned directly into another log where the tail of my right ski hit me directly in the back of the head. if i didnt have my helmet on i would have had a huge gash in the back of my head. instead i just got up and rode away injury free.
another thing, i can ski tight trees without my hat getting snagged and pulled. its nice just being able to look down and take a stick to the helmet instead of your face.