This past year, I've been using different battings in my quilts to see how they work, so when i'm done w/ the quilt, the extra batting(which is never much) is pitched. I don't want to combine a cotton batting w/ a cotton/poly batting into a project.
I quilt in a frame, so i also need extra backing to secure the quilt to the frame when I'm quilting the edges. sometimes i keep it, other times not. Again - it all depends...