I think I've hit my head twice in my snowboarding "history", once with and once without. Both times felt equally painful but left me with less than even a grade I concussion. However, I've hurt my shoulders, ribs, hips, shins, forearms more times than I can count. Even on rails, I've never even come close to hitting my head, it's always something else that hits.
The pros probably just don't wear it out of habit. Helmets aren't considerably irritating, uncomfortable, expensive, heavy or whatever else, so I have to imagine this is out of habit. Seb Toots wore a helmet in his part in Black Winter, I have to wonder if someone required him to wear one or if he just chooses to.