For me it's about the size of the scrap left. If you cut one 2.5 inch strip from a WOF quarter then you have a strip 6.5 left over. I can cut two strips 2.5 inches from a fat quarter to equal one WOF strip and still have a piece of fabric 13 inches wide. That's a lot more useful scrap size.
I have a friend that did art type quilts for a while and she pretty much only bought WOF quarters and when she quit doing art quilts I got a lot of her stash. It just isn't as useful and the scraps left from cutting squares is even less useful.