Could it be that we have been conditioned by garment companies relating to poly/cotton not being suitable for sleepwear. However, for me it is the issue of the fabric reacting differently when put together. The care that must be taken when pressing. I sent a box of fabric to a group one time that had some blends in it and they sorted the fabric and would not pay for the shipping of the part that had blended fabric in it. It is always amazing to me that someone would not make charity quilts from blends when they don't hesitate to buy sleepwear for their kids and grandkids.
We probably are going to see more blended fabric especially in the big box stores so will have to adjust our thinking.
I have tried using a combination of fabrics in a project and the frustration of working with it is not worth the aggrivation. I have a large supply of triangle shaped poly/cotton pieces that obviously came from a garment factory. I'm going to add some cotton strips between them and make large blocks for toronado quilts.