If you take off you engineering hat when you look at the situation it becomes less simple.
First off, having something stolen does not merely mean you now lack that object to most normal people. Theft is a violation of your property, and by extension, yourself. Having anything stolen carries with it psychological consequences; it is unpleasant. (And this is to ingore that for many people bikes serve several roles, some of which people may see helmets as necessary and some less so. Furthermore the options are not "bike with or without a helmet" but rather "bike with or without a helmet, or just avoid the mess and get there any other number of ways". When mandatory helmet laws are in effect, that is the decision people are making).
You are also attributing to much logic and planning to theives and vandals. The people who sell their wares online are the tip of the iceberg, damn close to the "criminal masterminds" of the bike crime world. The junkie who is just looking for crape to trade or give to a pawn shop doesn't give a shit what something is worth; if he can easily steal it, he will. And the punk kids who think it is fun to trash stuff don't really care either.
If you want to leave your helmet locked to your bike, more power to you. There is no way in hell I am going to do that in a city though.
First off, having something stolen does not merely mean you now lack that object to most normal people. Theft is a violation of your property, and by extension, yourself. Having anything stolen carries with it psychological consequences; it is unpleasant. (And this is to ingore that for many people bikes serve several roles, some of which people may see helmets as necessary and some less so. Furthermore the options are not "bike with or without a helmet" but rather "bike with or without a helmet, or just avoid the mess and get there any other number of ways". When mandatory helmet laws are in effect, that is the decision people are making).
You are also attributing to much logic and planning to theives and vandals. The people who sell their wares online are the tip of the iceberg, damn close to the "criminal masterminds" of the bike crime world. The junkie who is just looking for crape to trade or give to a pawn shop doesn't give a shit what something is worth; if he can easily steal it, he will. And the punk kids who think it is fun to trash stuff don't really care either.
If you want to leave your helmet locked to your bike, more power to you. There is no way in hell I am going to do that in a city though.