If you test how easily it breaks, manually, compared to another cotton 50 wt, you will see a difference. I use it for satin stitching (it's similar to a 60/2 embroidery thread) and to run stay stitches in seam allowances to check fraying (a row of Aurifil is way more subtle). But as long as I can snap it easily(and no, it's not old), I'll stick with what Harriet Hargrave recommends and not use it for piecing or for quilting that isn't very dense.
hugs,
Charlotte, who will undoubtedly need a flame-proof suit