> you have to carry the helmet, have it with you at all times, before and after riding.

You need it when you're on your bike, riding it. I commuted by bicycle for 19 years and biked (or walked) almost everywhere (Arlington, VA - Washington, DC). When I rode to the store or to meet someone or to get something to eat, the helmet usually stayed with the bike. I never thought someone would steal just my helmet. I didn't bother to lock it up, although you can get a wire to pass through the helmet and lock it up with the bike.