Absolutely - without question. A large group of us were at the gay New England ski week two weeks ago. One of us lost control in the glades and went head first in a tree. He had to be flown to a hospital. He doing fine thanks to his helmet.
Everyone on the slopes should be wearing a helmet. It should be mandatory.
Buy a Full Shell helmet. I love my Leedom
helmet. Consumer Reports ranked them for the best impact ski helmet a few years back when I brought mine. A good full shell helmet from them is their 'Scream' model which is what I have.