This other article talks up the benefits of XMPP. Yes, it is written by someone from Jabber, so read it with a grain of salt. Of course, the only remaining opposition is from the SIP/IM folks, and the former article pretty nicely pokes holes in that.
I guess to round out this discussion, something from the SIP/IM camp that picks apart XMPP would be good. Anyone have some pointers?