I've done that. I had leftover fabrics after making the top. I didn't want to purchase backing fabric so I started piecing the back and found when I was done I spent as much time on the back as the front. I gave it to my friend as a gift and she said it's one of her very favorites. So, I guess it was all worth it.
Top of Toni's quilt
[ATTACH=CONFIG]86402[/ATTACH]
Back of quilt
[ATTACH=CONFIG]86403[/ATTACH]