In Australia, helmet usage is mandatory ... it's the law, and I've heard stories over and over about some poor cycle-tourist who rode all day with a helmet, got to his/her campsite that night and decided to cycle the 1.5 kms to the store to pick up some groceries, without his/her helmet, and got ticketed for it.

Also many cycling events, and cycle touring packages, require that the cyclists use helmets. So if you're going to another country to participate in an event, and then do some touring as well, you've got to pack your helmet.