Why? E-Mail and Jabber work the same way. Everyone can run their own server, and many do, but most users prefer to use someone else's server(s).
Sure it can work that way but is that the ideal? Doesn't that make the network less robust and more vulnerable to attacks and manipulation?
Not really. If you're worried, you can always run your very own server.
What happens if some attackers start running a cluster of supernodes?
Nothing. Unless you happen to use one of those supernodes, which you don't have to, because you can run your own supernode. Or use a trusted one, much like people trust Google with their E-Mail.
The main point is why rely on this more vulnerable architecture when you don't have to? It isn't easier to implement.
It isn't easier to implement? I'd like to see some proof here.
Counterexample: The
totally distributed E-Mail system developed at Rice University is a hell of a lot more complex than running a (network of) semi-centralized mail server(s).
spaceshaker: yes I agree there will have to be nodes that act as proxies for mobile devices.
Um. That's exactly what a supernode server would do.