Most of the Dutch cycle at 15 mph on upright bicycles. No wonder they don't need helmets. For bicycle sports or even fitness rides I would say the helmet helps a lot. I was lucky to wear one two times.
Funny thing: wearing helmets is much more common in Germany. If you see adult cyclists wearing a helmets on upright bikes they'll speak German most of the time :)
Generally I wear a helmet if I'll be doing more than 15 mph, or riding through a city.