helmets are a really good idea... althought i don't always wear mine
just think of this... I was watching a big air comp. (everyone was wearing helmets) and I can remember the guys name but he slamed so hard that the helmet actual broke, not the strap the helmet itself, and came off his head completely knocking him out and giving him a serious concussion.. not that a concussion isn't something serious in the first place. now imagine if he wasn't wearing a helmet... can you say dead, in a coma.
this happened on nice snow... no rocks, trees, stumbs, or even ice