And if you look at the accident stats, survivability following a crash is significantly increased if the occupants are wearing helmets. I guess its a toss up between comfort and protection, I dont wear my leathers on my bike when its too hot-knowing full well if i fall off its gonna really hurt; but IMHO its worth the risk to not broil!
It would seem eminently sensible to wear a nomex suit and flying gloves, helmet etc in every aircraft, but for many that would detract from the experience.