If you go to the website for Superior Thread (www.superiorthread.com) they have an education tab. it will tell you everything you ever wanted to know about thread and then a whole lot more. Basically what I got from their site is that it's a personal preference. There is no right or wrong. According to their website if you ask alot of people why they use only cotton they will tell you that's how they learned and it was a tradition thing. Back in the day there wasn't any such thing as polyester or rayon. So, check out their website and I think you will find the right answer for YOU.