I use cotton because I use cotton fabric. Poly thread will eventually cut thru cotton fabric. I'm not spending that much on fabric and all the work that goes into a quilt to have it not last as long because I tried to cut corners and use a cheaper thread. If I use cheaper thread I might as well use the cheaper fabric as well. That's just my opinion; I want my quilts to last. I wouldn't worry about it so much with clothes because we don't expect them to last generations like we do quilts.