We use super fine, construction grade perlite, for cuttings at my work. Some of the elm, ficus, maple, and willow cuttings I repotted out of that stuff had the finest most profusely healthy root systems I've ever seen. I've since begun some experiments by repotting older trees in pure fine perlite. Its the same as pumice, less the high pressure steam puffing.
I think the difference with pumice vs other medias is the fact that it is naturally filled with air, so regardless of particulate size your roots will always have access to oxygen. With other medias its important to remove the fines because they clog the works, whereas perlite and pumice fines will still contribute to and aid root respiration.
