Helmets seem to be all or nothing. If there is a law forcing them, 95% of people wear helmets. If there isn't, 95% of people don't wear a helmet.
You would expect people to carefully consider it and make a decision on their own. For some reason people can easily rationalize not wearing a helmet in two seconds and roll with that for years.
I'm not sure about that. The law in Delaware is that you must have a helmet on your motorcycle for each person on it (I guess they don't want there to be a monetary cost to wearing a helmet.), and it seems like the majority (2/3?) of people I see wear them, but definitely not all.
To some extent it comes down to culture/how aware people are. I think in America, at least half of people would wear them regardless (and in a lot of states, there's either no law requiring them, or the law only applies to new riders).
there is an excellent episode of Top Gear in which Clarkson and co. ride across Vietnam on mopeds. The percentage of the motoring population which use motorcycles, rather than cars, is simply staggering. They throw out some interesting statistics as well (road deaths per capita 4x higher than in the states). http://www.youtube.com/watch?v=QMHVD-gWPDk
I suggest you suck it up and wear one regardless of the heat if you either: use your head for a living, or just value your life.