I move my coop every day. The one time I was away for the weekend, it was fine for a couple days in the same place.
I guess it just seems nicer for the girls to start off the day with a fresh area. By the end of the day, it is pretty poopy. I know chickens don't think like me, but I wouldn't want to spend too much time in a poopy patch of grass. I like to think they like the fresh area, too.