Lot of XMPP clients (Gajim, Kopete, Jitsi, Psi, Psi+, etc) are able to store
the bookmarks marking XMPP MUCs on the server (with optional bit whether the
MUC should be joined automatically on the connect to the account).
XEP for this functionality is XEP-0048.
I think this feature give a great liberty to the user that can choose and
change of XMPP client without having to copy all his bookmarks each time. It's
a basic and very useful feature of XMPP.