There is an open letter up on the Cocinella website about why IM clients which come bundled with support for multiple protocols are actually bad for interop.
I actually agree, but the problems listed about using transports as the solution are not merely theoretical.
I have been a Jabber user for more than six years and I have not found a single remotely hosted transport that I can rely on for the critical task of actually communicating with other people.
As far as I’m concerned, jabber.org should be hosting the most reliable transports on the planet and then making sure that other servers who want to run them can do so in a reliable way. There is nothing more likely to turn someone off than the promise of practical cross-protocol support turning out to be nothing more than wishful thinking.