The brain can only handle split imagery for 10 - 20 minutes then it starts to flip between eyes uncontrollably. The AH pilots handle this by turning the brightness up to the point where the non-hmd (Helmet Mounted Display) eye shuts down (pupil gets small). Of course they get wicked ass headaches. BUT if you go stereo then there a image alignment issues, the eye does not handle non-alignment images well, which would make the eyeglass mounting/holder not stiff enough or stable enough to mount a stereo display......ah all the issues come rushing back.......some day I should tell you about the display we designed for pit traders....
When I worked on the Space Suit HMD and the Army Helmet Mounted (Helmid) fighting the Sun was near impossible, causing us to raise the non-transmissiveness of the reflector (combiner) plate to 80 and 90% opaque. Making it rather difficult to see thought and causing me to use a non-see through combiner for the Army.
There are a host of issues, all of them well studied, which is why although this tech is like 15 years old, (I know I built this type of device before) I doubt it will go anywhere, all the advanges and apps I heard years ago.
But still some of the most fasinating stuff I ever worked.