I use sheets for quilting all the time. Some difficulties I have encountered are:
- sometimes they fray while piecing (the shinier fabrics seem to do this, for some reason...)
- sometimes the threadcount is too high and they are hard to push the needle through. That is a pain if you are hand piecing or hand quilting. It makes my machine angry. Also, if you make a mistake, you can't smooth the needle holes out!
On the other hand, sometimes they work great.
Usually I cut off a corner of the sheet and just play with the fabric a bit. If there is a problem with fraying, then I use it for a backing. If it is too dense, well, it just goes back in the cupboard until I can figure out what else to do with it!