I've used thrifted fleece blankets for batting a couple of times now. Once when I wanted the quilt to be warmer, I doubled it up with a thrifted flannel sheet. That's a very cozy quilt!
I sandwich and baste my quilts with Elmer's school glue, so I don't have a problem with stretching or shifting, and like someone else said, you don't have to worry about how far apart you quilt them.
Also, since it's not on the outside, any pilling that it might do isn't an issue.