On a more serious note, I've just never understood the need for a headrest. I can't believe one can have the ultra-fine motor skills, the hand-eye coordination, required for engraving and not be able to hold their head still enough to use a scope. And something touching my forehead would just be a distraction.
Not to mention, that's yet another ergonomic issue that needs perfectly adjusted, and STAY perfectly adjusted, to fit your face. Also, the field of a good stereoscope is big enough that I definitely notice myself shifting a bit to look at the far edges, especially when I'm engraving larger, or odd-shaped, objects that can't be exactly centered in my vise. YMMV.