Though it's hardly a new development. Seat belts have been mandatory for decades. People complain about safety restrictions like these at first but seat belt laws have been around long enough to reduce the complaints to a mere mumble. So may it happen with helmet laws.
I think there's too little data about road accidents publicly available. I think all documentation from police investigations of road accidents should be anonymized and made public after some time.
The main reason being given is seat belts for drivers and helmets for moped users. Info in Dutch from the official statistics bureau: http://www.cbs.nl/nl-NL/menu/themas/gezondheid-welzijn/publi...
Any policy which reduces the number of people who would otherwise choose to bicycle is de facto worse for people's health. If mandatory helmet laws decreases the number of bike trips by even one, it's a net loss.
I use a bicycle to commute to work every day. I think, I really should get a helmet. But I do not think they should be mandatory. I have been run over (more like bumped into) by a car more than once. But I was lucky and had no injuries at all. With my motorcycle it is a whole different story I had two real accidents and thanks to my helmet I am still able to write this.