The geek value of an eyeglass-mounted display is obvious. What's less obvious is that painting images on your eyeball with lasers instead of building large displays is much more environmentally-friendly, because it uses a small fraction of the power you otherwise need--all the light is going exactly where it's needed, rather than illuminating the rest of the room. A monochrome 800x600 display such as those made by Microvision can use 1 milliwatt or less, as opposed to the 15W of a standard 15" LCD, or the 50+W of a CRT. Also, one of the main causes of eyestrain is having to focus on a nearby object for extended periods of time rather than getting to look off into the distance, or change focal plane frequently. Such eyeball-painting displays can be set to any focal distance desired, and could presumably be built to change focal plane regularly, though none currently do so.
I think the benefit of heads-up displays goes way beyond that. The most important and obvious being, that we wouldn't be anymore chaind to our computer desks but we could use our coputer enywhere and at anytime. Actually, i think that in future we will carry our computers with us all the time -- just as we do with mobile phones thease days.
Considering the time we use next to our computers thease days, both cultural and economic consequences of being freed from our desks goes far beyond our imagination.