This is very context related. As an example, in the Netherlands almost no one wears a helmet while riding a bike and the cyclist union discourages the government from any legislation requiring people to do so.

Cycling is so safe, you really don't need one (unless you think you should also wear one walking down the street). The cyclist union's argument is that if you force people to wear a helmet, fewer people would cycle, more would drive a car and in the end more people would end up getting hit by a car.

That being said, I would 1000% wear a helmet in most other countries.