I am using cotton thread of a specific brand because I like it. I like the way it feels, performs, looks, and it's easy for me to obtain. I can also shop the sales and save some serious money on this brand. I have no trouble mixing my threads: cotton for piecing and polyester for quilting, or another combination. It's just personal preference, and I feel that any thread anyone wants to use is a personal choice and just fine. No Quilt Police here please!