I piece all my backs, have done for at least 20+ years.
I like using my stash
I like that both sides end up beautiful, although many have been simple, on the simple side the quilting shows up to advantage when I have that side up.
My backs seldom have anything in common with the fronts
I mean I don't try to coordinate anything
sometimes when I have a complicated quilt on the working wall, I'll piece a back from a stack of 10-12" block that I've cut to size over the year cleaning up scraps.
I've also used two tops...Bonnie Hunter mysteries for instance. Absolutely love them.
Half the storage space when not on the bed, and 1/2 the cost of having two quilts long armed...
In the end though, the only important thing is you are happy. Right?