I think it should be optional for adults.
I ride to work most days, and would continue wearing a helmet on my commute. I think there are too many idiots on the road not to.
But when I'm cruising up the beach, or the park, or on a quiet trail, there is just no need for me to wear a helmet. I am no more likely to fall off or have a collision than any pedestrian would - and pedestrian's don't have to wear helmets.
The argument that cyclists without helmets are more likely to get hurt, so should pay their own medical costs, is just plain dumb. With that logic, all rugby players should pay their own medical costs. And smokers. And fat people. And people who don't exercise.
Optional off road, compulsory on road, which is what i think you are getting at.