I often piece backs (have never bought wide backing - no particular reason). I try to coordinate the colors with the front when I can, but I also buy bolts of neutral colors when on sale and use that on all sorts of projects. I don’t lose sleep over what the backs look like and you’re right, with a little extra work it’s a great way to use up some orphan blocks.
That being said, many of my quilts wind up in some pretty chilly corners of the world and I like to splurge on minky or fleece. I have no misconceptions that my quilts are an heirloom masterpiece, but sometimes I do wonder about the longevity of these synthetic materials. We have yet to have 50 year old fleece quilts hanging around!