You loose half the resolution since each eye needs to be looking at the same thing. Second since you're looking at it in stereo you need to have the desktop a decent enough distance away from you to be able to focus on it or you'd go cross eyed.

I think this is what causes the effective resolution to drop to that much lower level.

Starting with a 2560x1440 mobile device, you'd need to get a very different head mount -- with a larger distance from the eyes to the screen for the same device -- to make that a useful 2560x1440 screen for coding or other non-stereo "flat" work vs. a pair of 1280x1440 screens for stereo 3D work. The latter is what you get from a phone-in-a-box VR displays with a 2560x1440 phone display.

Now, there might be a use for stereo 3D displays in programming, but I don't think anyone's developed and proven the tools that demonstrate this use.

You'd have eye strain from focusing on such a nearby object. In the headset itself each eye would only see half the screen.