After a couple bad falls in the park I got a helmet.
I ski, but I ride the parkprobably 40% of the time I'm on the mountain. At that, when I'm not in the park, I'm on the steeps taking up a couple thousand feet of elevation change in 10-12 minutes and those trees are scary.
One time I was on a narrow trail, caught an uncovered rock and slid into a grove of trees on a double diamond. Hit my forehead, but didn't get knocked and held onto the tree and barely avoided dropping into the well. Luckily my step brother was behind me and helped me get out of the mess and grab my yardsale. If I didn't have a helmet on, I think I could have been really really hurt.