I think it may be a pecking order thing, if you have perches at different heights, the 'top' chickens roost on the highest ones.....
But I also tend to assume that if they were really uncomfortable on the floor, they'd elbow their way onto the perches, as long as there's plenty of room?
Another reason for not using perches is the dreaded red mite which tends to lurk under the end of perches (good reason for having them removable) and crawl along and bite them in the night. Of course red mite can lurk anywhere, but it's worth checking out.