I hope they don't counter idiocy with too many specious arguments.

I can't imagine a bike where a mirror wouldn't be "particularly helpful" or a rider who couldn't find a single place to mount a mirror on helmet, sunglasses or bike. And as far as mirrors not being required elsewhere, riders of two-wheeled vehicles travel all the time between jurisdictions that require helmets and ones that don't, and people who choose not to wear helmets when they don't have to just keep one handy - strapped to their back seat or luggage rack on a moto, in a backpack or basket for a bici. A bici mirror is a whole lot easier to carry ... not that I would ever consider having a piece of safety equipment on my person and not using it.