That's definitely true for VR headsets like the Rift or Vive. I was thinking more along the lines of the Google Glass/Hololens form factor, which probably won't pose any problems with respect to weight.

In terms of focus, I don't think it will be much different compared to using stationary physical monitors since those usually stay at the same focal distance as well. The Glass/Hololens form factor only occupies a tiny part of your field of vision, allowing you to freely look elsewhere as you see fit (and AFAIK the Hololens, at least, has the ability to simulate different focal distances).

I'm picturing this as more of a supplement to physical monitors as opposed to a complete replacement in a typical workspace setting. What really excites me are the possible applications of something like this in mobile (an always-on, wireless second screen/HUD for your phone that takes voice commands or some other new control mechanism, for instance).