you use them on the bed because the high thread count keeps them from wearing out. the seams are on the edges, where they don't get too much wear. if you look at the seams closely, you can see that the needle has actually punctured the fabric to get through. a blend is much better to use for quilts if you make sure that all cottons have been prewashed and dried.
there used to be a thread (hehe) on this, but i don't see it anymore.