I've done pieced quilt backs too, for all the reasons Kalama mentioned. Plus another, for me the piecing is more enjoyable than the quilting so why not make something fun for the back? also, I don't really need double the amount of quilts in my house. And if it's decorative, like a wall hanging, I can flip sides when one sides gets boring. I make the back side more of a free design so it's not important whether it exactly lines up with specified borders. Otherwise trying to line it up when quilting would be a huge headache. By the time I'm done, sometimes I like the back as much or more than the front.