I contend that not putting this in the OS (and by this I mean a process dispatched to by [x]inetd) breaks 'small pieces loosely joined' (and I agree splj is a good thing). It also breaks abstraction and encapsulation, more good things.
Beyond this there are implementation pragmatics like what happens to a mesage that arrives and there's no app looking for it? If the target app is not running and the "networking" is in the app the bits fall on the floor. The the message router is at the OS level then it can spawn the receiving application.
It seems to me this is all about messaging. Even an RPC is a message, just with a guaranteed response. I'm not sure why we should treat an in-bound XML message of a certain "type" as anything other than a double click (single for KDE-types) on a document. What is it that you have in mind that's not a "message"?
Thanks for the pointer to rendezvous, I'll check it out.