I've used a few inches of voice grade ribbon cable to connect a router to a switch, running 100baseT successfully. However, if I tried 100 meters of that cable, it would surely not work. The cat6 certification means that I will get 1gb thru 100 meters of that cable, guaranteed. Cat5e might work, but not guaranteed. But I wouldn't want to bet on it, and an installer wouldn't either. Shorter lengths are more likely to work, but he'd still use the certified cable for everything.
