I have a small fixed run for their morning outtings. Then I collect the eggs and open the run to the free world. We're adding sand and mixing it around to keep it sanitary while they are in there. I see no problem with not moving the run... especially since they free range 75% of the day!? Besides, properly taken care of, a stationary run should be just fine, and plenty safe. Some people don't get to drag their chicken tractors around as they haven't the yard space, and they are trying to save their small lawns. Long as the run is the proper size, I don't see any issue.