Russ,
I am thinking about email-enabled applications in which the
multi-part message contains components which bear the authority
of distinct individuals. For example, a book with chapters that
are copyright by distinct persons. Digital signing of these
chapters prior to registration for copyright would allow each
chapter to be processed/confirmed separately. I suppose separate
messages are also possible and maybe it was the separate message
argument that made it acceptable not to provide separately
signed parts. Of course, forwarding of multiple, separately-
signed messages is another kind of example.
If we don't support the transport of separately signed objects
within a single carrying email message, then I imagine we will
look for alternative ways to carry bags of such signed objects
from place to place.
A Christmas tree message which is only partly readable by the
recipient, depending on what key was used to encrypt a particular
component is another example (which I think someone else already
suggested).
Vint