I use it a lot as backing. There are as many ways to make a quilt, as there are quilter's to make them...there are no real "Rules"...that's how I feel about quilts I will use or gift. If I were to do a quilt for a customer, then the customer would decide what to use...and if for a show...I'd probably use a fabric that was complementary to the quilt top.
I still buy muslin by the bolt...usually 108" wide.