I'm Dutch and do my daily commute on my casual bike without a helmet. In the weekends I ride my road bike and wear a helmet. I've once had an accident on my road bike and broke my helmet (along with my collar bone). A few weeks later I went out on my casual bike (not wearing a helmet) to buy a new helmet for my road bike. On my way back, holding the new helmet in my right hand, my steer got tangled up in the helmet and I fell. Undeniable conclusion: Having a helmet with you on a casual bike actually causes accidents!
For normal traffic use, people in the Netherlands use bikes where you sit in an upright position, and they don't go very fast (like 15 km/h). You can take quick evasive action away from the road, or put a foot on the ground to prevent a fall. It's hard to imagine breaking a helmet on a non-racing bike here.
I average around 20-22kph on my 8km commute riding my current bike. It's nothing particularly special, just an 8-speed city bike, and I certainly don't go all-out, I ride at what feels like a comfortable speed for the bike.
Biking in Copenhagen, I definitely see a fair share of people on granny bikes dawdling along, they're much more likely to crash due to sudden mechanical failure, or due to inattention from staring at their damn phones.
But either way, we're only talking about solo accidents. You can certainly also hit your head against the hood of a car, for instance.
Besides, I like wearing the helmet. It concentrates airflow across the top of my head, and it gives me more area to put stickers on.
However, I don't want it to be mandated by law. That has shown to keep a lot of people from cycling.