When I was living in Cape Town, I wore a helmet while climbing trad. and never questioned whether it was necessary or not. After all, if you've ever climbed a route on Lion's Head, the requirement for a helmet is set in stone.
Since I have moved to the UK, a new problem was developed, regarding helmets: space!
My wife and I brought our trad. helmets with us, from South Africa.
We also picked up a nasty addiction to skiing. Even though the last winter wasn't a good skiing season, my wife and I still managed a spot of great skiing in France. This, too, requires a helmet and, apparently, only skiing helmets are rated for skiing. (Note: you can rent helmets but, in our experience, renting skis and boots is fairly safe. Renting helmets is not always pleasant.)
Lastly, we both commute by bicycle, here, adding another requirement for another specialised helmet.
That's half a dozen helmets! We have more helmets than items of furniture in our apartment.