* Wearing a helmet only protects the head from a limited number of possible injuries.
* Wearing a helmet makes people overconfident and overly risk-taking.
* In the Netherlands, very few cyclists wear helmets because they are experienced by cycling almost daily, don't take unnecessary risks and street traffic design is far superior.
* Wearing a helmet looks dorky.
* Wearing a helmet is proven to cause vehicle drivers to respect bicyclists' space less, leading to more fatalities and injuries. [0,1]
Conclusion: A helmet is unnecessary for experienced cyclists who are only going 10-15 km/h in proven safe traffic-designed areas, all other use-cases: it depends.