i piece most of my backings- seldom use a (solid)...is that what you are refering to as 'plain'?
many times the back of my quilts look like a whole different quilt-
if i don't want to piece a back i tend to purchase 108" wide fabrics- still not 'plain'- usually a great print that will compliment the top.
once in a great while i will make a quilt that needs a muslin back- that's about the only time i use (plain)
just the way i do it- everyone does them the way they like-and i think it depends on location- and who taught you in the first place- i was taught to use as much as i have---and not shop for every little thing- if i can put together a good looking back for a quilt with scraps- i do.
and prints will hide any (flaws) in your quilting too- solids tend to show every stitch.