Owing to the behaviour of existing mail user agents, which
frequently treat even unrecognised subtypes of text as
attachments, text/paragraph SHOULD NOT be used as a main body
part in news or e-mail messages without prior indications of
suitability from the recipients.
...the idea being that at some point people may wish to include an
indication of the MIME-capability of their mail UA along with their
e-mail address and PGP public key...
Which is what the MAILCAP working group is working on. And also
see draft-ietf-fax-reporting-extensions-03.txt which has hit WG
last call. Not the best solution, pehaps, but easier to deploy
without directories.
-Dan Wing