phosphate media and a dsb are two entirely different things, the more contact the media has with the water, the more efficent I think it would be, and constant movement would eliminate dead spots on the media as well, it would kinda be like the difference between chasing after butterflies with a net, as opposed to laying the net on the ground as waiting for them to land on it, is it necessary, no, would it be a good thing to have, I'm thinking it would be, they are cheap for marine equipment, and they arent one trick ponies either.
You have me kinda confused on the forcing water through the substrate bit, its a media reactor, as far as I know it has nothing to do with the substrate