Exactly this, I ride with one as well as side mounted fixed mirrors. I can sweep 180 degrees to the rear with the glasses mounted mirror. Being 8" off the road on a recumbent trike I don't wear a helmet.My Blackburn mirror was indispensable while riding my recumbent, where the seating position prevented the view you would get from turning your head on an upright.
As for limited view, this is not correct. I could sweep the entire road behind me with the smallest movement of my head, something not possible with, say, a bar-end mirror.
The Blackburn mount is sufficiently strong that it won't be casually knocked off ( realignment after accidental movement is a pain initially) but it would not survive a crash or damage the helmet.
Absolutely right, anything protruding from the helmet is a potential hazard. The most inocouos event can cause an off.My problem with helmet mirrors - and especially cameras - is getting them caught on something
During the summer I got a twig off a hedge jammed into my helmet going downhill on a road - my head was jerked backwards, which luckily changed the angles and it slipped out.
I was damned lucky to stay upright - and lucky no car was passing me at the time
So - basically - I am against anything that sticks out from my helmet - especially as I ride on paths and tracks a lot and there are sometimes overhanging branches which brush my head
If I wanted a mirror I would get on that goes on the end of the bars
+1Nowt attached to my helmet.