I reckon that a distinction should be made as the type of work involved. Most pilots in Australia will use a helmet for airial work applications. For offshore, police or emergency work it is also the norm.
There is often some contention as to whether a scenic flight or general charter pilot should wear a helmet. The thinking is that it may scare the **** out of the passengers if they see the driver wearing a bone dome. I tend to agree. but I guess accidents do happen.
Personally I don't wear a helmet on a normal passenger carrying charter, but find it uncomfortable not to wear one when I have a door off.
As to the temperature issue, we were often working in 45 degree heat chasing cows around and I never once thought that I would be more comfortable without the helmet on. Obviously the type of helmet plays a vital role in this.
Some contractual jobs require the passengers to be wearing helmets. On large government contracts the pax are issued with their own, and it is then 'unusual' if you don't wear one.
Purplepitot - No disrespect intended, but everyone 'plans' not to crash in the first place. Some day, you and I may not have the choice.
With that in mind, if a scenic pilot wants to wear a helmet then go for it.
cheers