A reasonable number of people DO have stuff in the run for chickens to get up onto. As noted, it is only a good idea IF you have a top on your run and many BYCers don't, which is probably part of the reason you don't see it mroe often; also in a small run (which, again, is what many BYCers have) you can't install much in the way of 'habitat complexity' without making it quite difficult for a person to get around in there to clean etc;
and the biggest drawback IMHO is that it makes it a lot more work in many circumstances to get the chickens to spend their nights indoors during the summer, where it is safer. It can require you to do a lot of 'training' and manual relocation and so forth, which not everyone is willing to do (especially if htey're not home and are relying on an auto door).
So it's a tradeoff, IMHO, and more useful in some situations than others.
Pat