As with everything else in our quilting world, there are no quilt police and it is personal preference. I only use cotton in piecing and quilting with an occasional rayon embroidery thread for quilting. I also find that if the tension is to tight top or bottom that the thread will break when pulled. Also with any of the stretch type materials you need to have a 'give' in the thread. This is where the new machines have it over the older ones---they have built in stitches to use on these fabrics. The straight stitch that is built in for stretch machines goes forward 2 stitches, back one, forward 2, etc which makes the stitching stronger and able to stretch instead of snap.
Also, your question on using poly for piecing--I think it may have to do with the weight, but like I said above, I don't use it when making quilts so am not sure.