The theory is that the thread count is to high. My thought is that thread count varies with the sheet you choose. I have used them at times without any problem for backing even for hand quilting. I look for the lowest thread count I can find. Sheets are not usually all cotton so for a purest that would be a problem. As far a using in the top it would depend on your skill level. Cotton has more "give" to it and is easier for beginners to work with. Blends last longer for quilts that are to be used and not just for show. The quilt purists (quilt police) will probably disagree with me, but I think you can use anything you are comfortable with.