Steve Allen's book says that in the technical canyoneering section. It doesn't give beta for technical canyons, but goes as far as alluding to the fact that only canyoneers with a death wish use a sling that was previously placed by someone else. Maybe in some dedicated sport or trad climbing areas, but this is often not the case. The Hourglass on Little Bear is a good example: Climbers are always leaving a big rat's nest of ropes, slings, biners, etc. on that route. The above is typical, sometimes it is worse than that. What's scary is that people use them from the bottom without testing them. There are actually many routes that have rat's nest like the above. Apparently some climbers in Colorado aren't afraid to do a climb, but they are afraid to rappel off, or so it seems by looking at the rat's nest rappel anchors. A good example is the one on top of Devils Thumb. I filled my whole pack with the rat's nest. In the Fisher Towers, for example, it is also customary(?) to leave fixed fixed ropes behind in order to come back later and climb the next set of pitches on a route. Fixed handlines and slings are also left on approaches. It's more than canyoneers that do it. When it comes to rope grooves though, I would venture to guess that by percentage, canyoneers are more guilty. Part of this reason is that canyoneers often look for canyons in soft rock, while many rock climbers prefer harder rocks. Still, when climbers do climb soft rock, they leave rope grooves behind. There are plenty of rope grooves on the sandstone towers such as the The Penguins, Independence Monument, many of the Fisher Towers, etc.