I've never understood the rationale for putting the flimsiest part on the furthest outside of your bike where it is most prone to breakage. If you don't believe me, how many times do you just lean your bike up against something and lock it rather than hunt around for a proper stand to lock it up. Doesn't take much to knock a mirror off. Some people do it for "fun". With a helmet mirror, you generally carry your helmet into a store with you. If the mirror gets readjusted it's easy to set it up again to see your left ear, which is the base position. A couple of tweaks and you're ready to ride.