Car drivers often get head and brain injuries if they are involved in a collision with another car or with a tree/building. Do you wear a helmet when you drive? The helmet doesn't add almost any inconvenience, is reasonably cheap and light, as you say.
Unless you're on a racetrack or driving a test vehicle, probably not.
The risk/inconvenience ratio seems OK for me when driving a car without a helmet but not for biking without a helmet. I'm not advocating for mandatory helmets. But I would feel very uncomfortable if they would be banned.
In Europe most people didn't even wear helmets when they went skiing (on-piste) until a few years ago. Now pretty much everyone does, but it's one more thing you'll carry with you everywhere you go. And on warm days I sometimes have to take my helmet off in the gondola because it gets very warm.