Let us not overlook that the the affects of concussion are cumulative. Just because you 'feel fine' after a day or two, doesn't mean that permanent damage was not done. Subsequent head injuries will run the risk of creating greater damage than if they were the first injury sustained.
Better helmets are available. They aren't all that stylish, but they exist. Racers use helmets specifically designed for racing and are, IMHO, better than the helmets with soft sides, built in speakers etc for protecting the brain. Many racers, especially young racers, wear mouth guards to protect from the type of concussion referred above as 'dental concussion'. Racers that experience a concussion are required by race officials to provide a doctor's report that they are capable of competing again. Baseline testing is done every year for junior racers to be used to determine their race-readiness after a concussion.
I wear a racing helmet anytime I am skiing at a resort. I wear a racing helmet when I ski BC. I have worn motorcyle helmets as protection when ski racing when that was all that was available (mid-70s). A full Bell helmet saved my brain.
The fiberglass shell of the helmet was chipped on impact, visible in the picture. We didn't use mouth guards back then but that probably would have helped as I landed on my head, compressing vertebrae T3, T4 and breaking a few teeth in the process.
As a coach, I've had to deal with racers with head injuries all too frequently. They, of course, had a racing helmet on so I can only image what their injuries would be like with lesser helmets or none at all.
I won't argue with ChrisFromRI that ski racers are the best skiers in the world. That has been discussed to death in other threads with no clear winners or losers. I will say that labeling a ski racer as nothing but a ski racer is often inaccurate. Not all ski racers stay in the race arena.
Wearing a helmet does do good. It doesn't prevent traumas elsewhere to the body but it increases the odds that injuries to the brain will be less severe. As RR points out, you can survive a knee injury even if it requires amputation. ACL reconstructions cost in the 5 digits. A brain injury may cost nothing, especially if not diagnosed, but sustaining a brain injured patient that is unable to take care of himself will cost into 6 and 7 digits depending on how long the patient survives and the level of treatment required.
Life is all about dealing with risks. For some, like ski racers and people that like to ski for the adrenaline rush of speed and skiing in the trees and steeps, helmets ought to be an essential piece of equipment. For groomer skiers that don't push the limits, they certainly can help reduce or avoid injury in collisions or simple falls but could be considered optional. You can't really count on being in control all of the time and when that collision from behind or that ice you didn't notice trips you up, you will probably be glad you had a helmet on after you head hits something hard.