Maybe some users (well, not maybe, as I've had multiple complaints) like having a choice over which messages they receive HTML and which they don't. I shared the opinions expressed here with the most prominent of the reporters within my organization and he stated that if he totally disabled HTML, that the few mails he would like to see as HTML would need to be modified. Some people in the org wouldn't mind me stripping HTML at the bridgehead SMTP level, and some people in the org love it (like myself.)
Even if a message is RFC 1521 compliant in it's implementation of multipart/alternative content, doesn't mean it's necessarily reliably renderable across all clients either. Some HTML based e-mail services really mangle HTML messages.
And for the really extreme folks, an RFC 1521 multipart/alternative message has both text/html and text/plain, including markup and graphical content that is unessential for text clients. It's larger, requires more bits to be passed, which ultimately is more electricity, which causes more global warming
Pardon the RFC references though... I was an Engineer for the worlds largest single-site Exchange implementation, and world's largest IMAP implementation. The mail stuff has hind of stuck with me.
