Jeff notes:
... different body parts to be encrypted for different principals.
I can envision applications where someone might send me a multi-part
message where some of the parts are encrypted, but not for me.
Instead I forward these parts (perhaps with additional information)
to other parties who can decrypt them.
This is an entirely credible scenario. One should be able to delegate
virtual papershuffling chores without worrying about the clerk steaming
the envelopes or jewelry boxes open just because all arrived in the same
shipment and he's allowed to read the cover letter.