Sorry to be blunt but its a case of:
If you don't think you need a helmet, then you probably don't !!!
ie, Have you got anything you (or significant others) would consider worth protecting?
Our kids have always worn them but, to be honest, the penny only dropped for me about 5 years ago. Since then the wife and I have worn them too. Now I'd feel vulnerable without. I've seen too many accidents where it would have made all the difference between laughing it off and being carried off.
I'd say the chances of somebody else hitting you are much higher on skis than when you're on a bike.
You're probably also likely to be going much faster on skis than on wheels.
Last year, one of the kids in our party took a big fall, failing spectacularly to land a massive jump (maybe 20m+)
She went down HARD on her back and hit the back of her head in the process.
Her helmet definately served its purpose but unfortunately she was evacuated for suspected spinal. (Thankfully later given the all clear)
My eldest is now after a spine protector for christmas as he's still narked that she outjumped him (even if she didn't land it).