I've heard of resorts where helmets are mandatory for everyone. I know in all the ski programs here locally, (I'm in Colorado) children must wear helmets. They have been coming on for about 10ish years now. I haven't been on the slopes much since then and I don't remember anyone wearing them then and now must people I see have them on. Helmets are mandatory for my family. DD1 and DH are avid boarders and they always wear them, DD2 has been up a couple times this and wears one.
A couple of personal stories from an active skiing/boarding family. A tourist last X-Mas day year went sledding with her children near here, did not wear a helmet, ended up hitting a pole, died of head injuries upon impact. Who knows if a helmet would of saved her but she might of had a chance. My sister in high school before helmets fell while boarding ended up slashing her scalp open, it took over 20 stiches to close, a helmet would of prevented that. My dad fell a couple of years ago during early season conditions and actually cracked his helmet open on a rock, he was ok. His best friend fell last year skiing and ended up with stitches in his head as well, he was not wearing a helmet. Needless to say, helmets are a must IMO.
Safety has continued over the years and with the number of skiing deaths that due occur with collusions between other skiers and trees, helmets improve safety conditions. I have no links saved to back it up right now, and my children are screaming for me to make cookies with them but I bet a quick google search might bring something up. It is also like skateboarding and ice skating, years ago no one wore helmets, now days many do. No one used to use seat belts or car seats either. Same thing, a skate boarder fell a couple years here in town, no helmet, died on impact, they said a helmet probably would of saved his life.