Definitely not as a potting mix, but I think they may be useful for propagation. You could actually have a test, where you compare growth in leftover pumice, and growth in no soil, or large pumice.

I use leftover Turface (real fines) and leftover small pumice for rooting -- sans leaves mostly. I don't moisten, I don't water….I just stick leaves into the medium and be patient. Sometimes it takes months. Sometimes the leaves don't root -- but sometimes they DO, and then they get potted up into a real mix and watered accordingly. Gotta' have patience though -- you can't go potting them and pulling them up every week. I rooted a Sans Boncel plant from a single broken leaf in Turface fines. It took nearly 5 months….but finally a little tiny pup started raising its head.

Thanks for the input guys! I'll test propagating leaves.