I think I've found the most compelling argument against wearing bicycle helmets: Zombies don't wear them.
You'd think that the undead would take the most basic precautions to avoid being dead again, and yet there they are, shambling around without protection for that vital brain case. Clearly they have made this decision because the wearing of bicycle helmets is an intrusion on their personal space, an infringement on their right to choose and finally, few bicycle helmets come in the appropriate zombie colours.
If it's good enough for the zombie apocalypse, it's good enough for the cycling apocalypse. Join the undead.
(Photographic evidence of widespread zombie adoption of head protection is welcome, should anyone wish to contradict this premise. Sporadic helmet use by zombies should not be considered an indication of a widespread trend or an indication of a clearly considered choice intended to extend their unlives.)