Please point to an example of a useful multipart message seen in
this list or that might someday be useful in this mailing list.
I have sent to wg lists a multipart containing a preamble and an
internet-draft or similar file. This makes it easy for recipients to
save the draft as-is. Sometimes I have made the file a
content/external-body to avoid stuffing O(100kB) into many mailboxes
where it wouldn't be wanted.
You said "this list", but the above scenario would apply if the
document were pertinent to a BOF or brand-new working group.