I often use sheets for backing a quilt. There are no seams to mess with, and there are certainly lots of color choices. The sheets hold up well with washing and drying, and I've never had any complaints from those who've received a quilt from me.
Occasionally there will be a need to use some sheets in the quilt top itself. It's always worked for me. Maybe I'm just not that particular, but I've never had any problems. Historically, quilts were made using what was available, from denims and corduroys to silks and velvets. Some of the most beautiful historical quilts often had fabrics that would not be "approved" by many quilt police.
I say: "Use what you've got and whatever makes you happy!"