Chad,
I see your point, but the fact is, there ARE some cases in which using the eye would work. Take counter top, vanity top, and any solid surface (corian, formica etc.) fabricator and installer. Most of them go to a site where they have to install something, make a template out of cardboard, wood, coroplast etc, take the template to the shop and make the counter top out of that template. No CNC no CAM no CAD involved.
Now If you have a cnc router, equiped with mach, a 10$ webcam, and that plug in, you can lay your template on the router, have mach trace/learn/scan the template, now you have a CAD rendition, which you can then use to lay out sink cut outs, engravings, drain boards, etc. and then use your cam to generate an output and cut in mach with your router.
Why not use just site measurements, and use those to draw up in cad? Well, as any contractor would tell you, there is no such thing as a straight wall. difficult thing to measure by hand and then draw in cad.
THey sell some systems (Phototop i think it is called) that have some sticker markers that you lay out on the install site, use a calibrated special camera, take lots of pictures of those markers, post process those pictures at the shop on their software and then export to DXF and cut, it is a hassle, you have to be trained very well to get good results, plus it costs over $10,000 last I remember. It costs more than my 12'x12' router im building!
SO there are some occations that it will come in handy.
Regards
Fernando
PS. some company even sells a template digitizer, its a 2 axis hand operated machine with alaser pointer for spindle, its tilted almost vertical, you have to tape or fix your hand made template on the machine, and then hand trace the template clicking a buton on all the features and telling the machine if it is a line, or a 3 point arc, etc. Think of it as a HUGE drafting tablet.