It's fine to use poly/cottons in quilts as long as you understand how they differ from all-cotton. As someone else mentioned, poly/cottons are more slippery to deal with. The biggest issues are that (1) they wear differently, as quiltsRfun mentioned with the binding, and (2) they fade differently. If you mix the two types of fabric in a quilt top, the cottons will fade over time while the poly/cottons will retain their original bright colors. This can create quite an unappealing quilt top over time. This is the major reason why I would use either all cotton fabrics in a quilt top, or all poly/cotton fabrics in a quilt top.