I think it all comes down to personal preference although some people don't mix poly with cotton due to possible non-shrinkage of the poly. I don't like working with some polys because of their smell, but that's just me. I sometimes use it for the color or pattern. I also sometimes use poly thread for quilting because I want a matching color and there isn't anything close enough in cotton as my LQS. Do your own thing - and enjoy it.