I think Rhys has put it all quite well. In particular:
I've been a long-time champion of text/enriched, but I now think that
maybe we should depreciate it in favour of HTML, since that seems to be
more popular these days, and is more flexible. Except for a few example
MIME messages, and stuff from Nathaniel, I don't think I've every received
any text/enriched (or the earlier text/richtext) messages, let alone
text/simpletext messages. (I haven't received any HTML messages either,
but you know what I mean :-) ). What do others think? Too hard? Worth
pursuing? Couldn't care less?
(The only thing I'd point out here is that if you had other
correspondents who were Andrew users, you'd receive more text/richtext,
but that's not a big deal.)
I think that if HTML had been around when MIME was first being drafted,
text/richtext and text/enriched would probably never have been invented.
It's certainly fine for the same role. Does anyone have any
*standalone* software for viewing HTML?