I sometimes use old clothes or other fabrics, but I will say....
I would not use them if I am making a quilt intended to become an heirloom. Fabric does wear out with washing and wearing, and if I am going to hand quilt a special quilt, I will use all new, good quality fabrics in it. The same applies for quilts I make to sell.
But for 90% of the quilts I make, I don't mind used fabrics.