From this comment I found on line, I wouldn't want to use bobbin thread in my quilting: "Bobbin thread is a fine thread intended only for use in the bobbin during embroidery. It is not a strong thread, so don’t use it for regular sewing. This very fine thread reduces the build-up of thread under your embroidery, reducing bulk, and making your design."
It was at this URL: softer.
http://www.meissnersewing.com/Meisne...ksJune2008.pdf