When I did my own darkroom work back in the day, I would do some pixel peeping (or like Mark says above, grain peeping), just to know how much leeway I had for enlargements. But the vast majority of my past photography consisted of shooting, sending off the film, and getting back my 3x5's or 4x6's. At that size, almost everything looked good. Now when I look through my images in Bridge, I use the loupe at 100% and if they don't look sharp, they're usually gone. If I know I won't be cropping much or at all, I may keep them if they are otherwise good images.
The difference between then and now is, of course, the equipment. I used to use bodies and glass that would be considered consumer level today. And of course everything was manual. When I manual focus today, even with my Eg-S screen, I never get the level of sharpness that the AF will, unless I take the time to use a tripod and LV at 10x zoom.
Five or ten years from now we'll have cameras that do what Gigapan does today and they'll hang easily around your neck. Then we'll see some gigapixel peeping.