And apparently nobody on a bike has a bad neck!
I have a bad neck, but it'd need to be really quite bad to stop me looking behind, probably enough that I'd be on a recumbent tricycle and need a mirror anyway, to see around the seat back.
The only time I feel like a mirror would be useful to me on an upright is when I'm guiding group rides and wanting to count that everyone's still following, but the tail-ender ought to send a message forwards if there's a split or problem anyway. It might be more useful on the road bike than the others (roadster, hybrid, MTB...) because the more leant-forwards position and twitchier steering reduce how long I can look back stably.