I've only been skiing for 20 years and for 17 of then I never wore a helmet either. 3 years ago I bought my first and honestly I couldn't go back.
First : They keep you warm :-). I usually start skiing in early december when it's quite cold, -15 is not rare with the wind chill factor.
Second : I've had my fair share of falls, knocks and bruises and still suffer from a shoulder problem that dates back about 16 years ago after falling on an icy piste, fortunately it was my shoulder and not my head because I know for sure that a woolly hat wouldn't have absorbed much shock....
Third : I ski with some excellent skiers and all of them wear helmets, they know only too well that at the speed they ski, accidents can and sometimes do happen and that the helmet could potentially save them from very difficult situations. No, it wont save them in all situations but even if though have a 80% chance of avoid major head injuries then that's always a plus.
Fourth : Skies, boots, piste grooming and technology have improved too such an extent that even beginners can ski much faster than they did many years ago. On top of that the pistes are busier, hence the likelihood of more accidents... A little bit of extra protection is not a bad thing.
Fifth : They do not remove any of the sensation, you can still hear well and you almost don't feel them on your head.
You should try one one day, you will be pleasantly surprised to learn that they are not a gadget/gimmick or fashion statement. And they are not even that expensive compared to the price of the helicopter ride :-)