Gosh, your question really made me stop and think. When I first started quilting many years ago, I always used muslin for the backing- mainly because it was what my grandmother had always used, and because it was inexpensive. Then I started using quilting fabric for "special" quilts, like ones that were gifts, or for a bedspread that the top would be turned back and the backing would show. These days I use quilt fabric exclusively for backing on every quilt I make, but I have such a large amount of fabric that lately I have been thinking I would start piecing the backing to use up fabric.
But your question made me think about using muslin again, and now I think I probably will. Why not?