Part of it's practicality. In NYC we now have a bike share - grab a bike from any station, ride around, return to any other station. Frequently I'm out and about and feel the need to grab a bike and do something - but I don't because I don't have my helmet with me.

At the same time, I really don't want to have to lug my helmet around everywhere just in case I want to take a short ride.

Having a compact "helmet" all the time that takes up next to no space (and is inconspicuous to boot) would be very nice.

When cycling is a sport/deliberate activity, rather than a transportation method/incidental activity, the dynamic changes.