Heavyweight RSS hurts clients, too. (Especially lightweight ones.)
Chris DeSalvo points out that RSS bandwidth requirements hurt clients, too, especially when those clients are bandwidth-limited (by rate, cost, or both). His example (natch) is the Danger HipTop. Which, of course, makes me wonder what kind of mindblowing apps you could build on the HipTop if you could sneak a p2p overlay onto it. [It’s not out of the question, either, since the HT is a Java device (well, J2ME anyway).]
Remember, peer-to-peer isn’t just about file sharing; perhaps most interestingly for mobile devices, group messaging (self-organizing, scalable multicast). The possibilities are endless!