i'm a far better ski'er than i am snowboarder. i just enjoy snowboarding more. it didn't used to be cool to ski, but now it's coming back around.
basically, unless you are going on hills far below your ability and just chill, you should wear a helmet.
if you are new, young, changing equipment, on a new hill or doing some crazy shit, it's common sense that you need protective gear.
people don't understand that frozen snow (ie ice) is just as hard as concrete, and is extremely sharp. even if you don't get injured, a fall to the face can rip your skin off.