Well, I would say it's not stupid. It has some practical use, it's not dangerous and it doesn't make you or the helmet look silly.
Now, whether it's useful or not to a particular rider is up to that rider.
Riders have been battling with their mirrors for some time now. Many times, they're hard to see with because your shoulders get in the way of the view. If you get longer stems, there's the potential for more vibration, which makes visibility a problem. Also, if the mirrors are set out too far, you have to actively look over at them to use them, rather than just making a quick glance. Finally, while it's easy enough to see back on either side of the bike, another rider sitting directly behind you can be blocked by your body or the angle of the mirrors.
This eliminates all those issues. It's likely easier to glance at, is going to vibrate with your head, rather than independent of it (so your vision syncs up with it) and it's not blocked by anything other than potentially a passenger. It might take some getting used to in order to avoid getting distracted by the movement just at the edge of your field of vision, but that comes with anything you add to the bike, like a new tach, a GPS or some other device you tend to want to look at.
So, here's my assessment:
It's good if you:
- have a limited rearward view due to the size or placement of the mirrors
- have mirrors that are set out far enough. their proximity requires too much of a shift of vision to check them properly
- have mirrors that simply vibrate too much to see clearly
- want to be assured of an unimpeded view to the rear
- want to easily to check up on your passenger, especially when carrying kids
It's useless if you:
- often carry a passenger and want to use it to see what is behind the bike
- have easy to see mirrors that show a proper view
- find such things distracting
- can't afford the helmet it comes in, since there's not add-on for other helmets