Helmets with built-in light & mirror mounts?
Why not? Many 1,000's of cyclists use helmet-mounted lights and/or mirrors. Yes some lights have a mounting strap for helmets but it's a little bit of hassle to remove or mount esp in the dark and/or cold. Having a quick-release mechanism would be nice. Esp since we now have efficient small LED lights that are sweet for helmet-mount use.
But perhaps more important, a helmet mount for mirrors would be super. Many riders mount the mirror to sunglasses but of course then if one rides at night or in low light the rider has to continue using the glasses, perhaps having to switch lenses (another hassle).
I now use the EVT Safe Zone Helmet Mirror (purchased from excellent Mt Airy/College Park Bicycles) & it's fantastic. Huge area that obviates having to look behind. Heh, track sprint racers should use these since they spend half of their races snapping their necks to check behind! Only problem is that it relies on zip-ties for mounting which isn't too solid. A little nudge can mess up the aim requiring some fiddling to adjust. A little built-in mounting bracket would eliminate that problem.
Try to purchase a car without left/center/right rear-view mirrors & no lights!? But bikers, we're left on our own to improvise.