I don't allow HTML or attachments in my lists, either. People get an
autoresponder explaining why, and a copy of their message to try cleaning up
and submitting again.
HTML simply doesn't work in Digests, and doesn't add much elsewhere.
Most people who use it aren't doing so out of some kind of principled
visionary commitment to new media, but simply because their mail software
comes configured that way by default and they don't know how to turn it off,
or even that it's possible to do so. A couple of list bounces motivates
them to learn.
I would not object to HTML as an option if broadly available mail clients
(or middleware) made it optional for Digest or individual message
recipients, but we ain't there yet.
I do participate on a couple of lists and newsgroups where HTML is allowed,
and on those groups it is invariably the rule that the postings containing
useful info that I care about, were sent in plain text. Funny about that!
I also don't allow Subject lines of the form "Re: Beekeeping Digest V4
#131." An autoresponder explains why, and encourages submitters to use an
appropriate Subject line.