Steve,
One of the things you didn't emphasize is the current 1421 spec does
provide for alternation of enhanced and non-enhanced body parts, so it
would be possible to cobble together a user agent that generated a
single message with different enhancements. I don't know of any user
agents capable to generating such messages very easily, and I'm pretty
sure that most user agents would not know how to parse a message that
has alternating enhanced and unenhanced sections.
Handling of multiple body parts is native to MIME on the other hand,
so I have hopes that it will be far more natural to deal with multiple
enhancements within MIME than it is outside of MIME. MIME also
provides explicit support for encapsulating of messages (as distinct
from message bodies) and for recursive encapsulation. How well all
this works out in practice remains to be seen, of course.
A different question is whether it's possible to build and distribute
a MIME-PEM implementation that does not require full MIME support and
then fits into the same environment that a 1421 implementation would
fit into. If so, then I think the MIME-PEM spec is a fair candidate
to replace 1421 entirely.
Steve