For me, it's all about keeping some control over technology. I tell a very limited program like mutt what to do, and it does it. This is in contrast to begging an incredibly complicated program like a browser to do something, where much of what it is doing is controlled by invisible embedded code written by people who may be hostile toward me.
That's fair enough, but you are very much in a minority.
I can tell you that on just about every metric - brand recall, clickthrough rates, ROI, unsubscribes - HTML outperformed plain text. There were exceptions, both in terms of types of campaign and types of sender - say, if your list includes lots of Hacker News readers.
We found, for example, that including just one image will increase open rates by more than 50% but I can't find it now. Another article on the same topic is here - http://www.alchemyworx.com/alchemy_worx/2009/newsletter/issu....
It's boring to see everything being reinvented again, just with more color and horrible mistakes.
A quick note to web.de and gmx.whatever who send mails as HTML-only: you suck
Also, text-only doesn't mean monospace. I'm happy to read email in whatever sans-serif font the email client picks, with nice whitespacing.
Fortunately, most e-mail clients support a "show this e-mail in text only" view mode. This means that you can have it your way, and others can have it theirs!
Either you will be sending in HTML or plain text, I feel there shouldn't be any excuse of not attaching plain text version of it.