I think that it's all about finding a helmet that is comfortable, light weight, and low profile. I use a Smith Vantage, and I had two sent to my house. When I picked up the box, with TWO helmets inside, it felt practically empty. I feel like a lot of the helmets designed for the "park look" (like the skate helmet look) are heavier and lack ventilation.
I also know tons of people who wear thin beanies under their helmets for warmth. That's where the inner adjustable helmets are nice.
Unless you are jibbing or riding icy park conditions, I think riding in the trees is more dangerous for your dome piece.
Have you ever actually hit your head hard while riding? I know more than a few kids who never wore helmets for the same reasons you are saying, only to slam their face on something and get carted down the hill. Needless to say they ALL made peace with some dome protection.