These are wearable computers, not glasses with visual powers that are not yet feasible to implement, period (I'm guessing there would be a few technical issues with head-mounted-x-ray machines).
All the glasses could do is display a layout of a known engine, they can't see inside it to highlight a problem. So, it seems like they would be more of an overall nuisance than anything else in that scenario.
For example, imagine inspecting a plane's fuel tanks from the inside. You could bring a manual, but getting it out of your pocket may be cumbersome, checking that you took it out after the job takes time, etc (forgetting your glasses is less of a risk; you would need them soon again. That manual? The next tank might have a different 200 page one)
And even if it was a nuisance once you learned the engine, it could be a useful tool in schools or someone learning a new engine.
Even imagine a small engine machine shop that sees 10s or 100's of different kinds of engines.
http://www.youtube.com/watch?feature=player_embedded&v=i...!
It uses 3d tracking and could easily be changed from an AC unit to an engine. I think the software is definitely ready.