I think you are missing the point. When YOU mow the lawn or when YOU climb the ladder you have control over what YOU are doing and can take precautions. If you are riding a bike in the street, it's the OTHER guy in the car (or other obstacle) you have no control over. The helmet IS your precaution!

To be honest, I have always rode a bike without a helmet since I was a kid. It just never seemed that dangerous. I'm not going very fast. I'm usually NOT in traffic. I do however ALWAYS wear one while on a motorcycle. Only recently did I make a purchase of a bicycle helmet. Now I won't sweat it if I do ride in traffic (although I still tend not to). As an adult, it's your choice (if the law allows it in your state).