I don't think the analogy quite holds. Native SIP is both, I think (like
HTTP). In any event, in CPIM gatewaying, protocol translation would be
done on the SIMPLE-XMPP level, and the MIME bodies present within SIMPLE
or XMPP would be relayed without modification. Since SIMPLE and XMPP both
have headers and whatnot, that protocol translation of their metadata is
non-trivial.
As I said before, MSGFMT does not seem to confer any properties when it is
used without security. A gateway still needs to perform protocol
translation on some of the SIMPLE/XMPP metadata in order to be able to
route messages. The metadata it needs to be able to translate is roughly
the same as what MSGFMT will contain. Carrying this metadata around in
MSGFMT without applying security properties to it will always be
redundant, I think. What purpose do you envision it will serve?
avoiding nontrivial protocol translations seems to me like a worthwhile
purpose in itself.