Firstly, this isn't an attempt at conversion. It's here becaue I keep getting asked why I tend to send and prefer to read mail in plain text. I don't really mind what you send mail in, so long as you accept that I'm not near an html-capable mail reader very often and if there's no plain text part I'll just not read it until I am, and that I treat html email as I do websites - if I don't like the look of it, I don't read it.
When you send me an email, all I want is the words. I've spent much time experimenting with different fonts, point sizes and colour combinations and I know what works for me. It is ~7pt white monospace on dark grey. I don't particularly like having to read emails in other formats, particularly not the current vogue of sans-serif black on white, and if I'm honest I don't see why I should - I've don't recall ever recieving an email where the font face or colour scheme bore much of an effect on my understanding of the contents beyond my ability to read it comfortably.
Similarly, I accept that you probably don't want to read your emails as if it's some kind of inverse midget typewriter, and I will never send you an email that requires you to. If I send you an email, I will send you nothing more than a string of words, and leave it entirely up to you (and the configuration of whatever you choose to read your mail with) do dictate what it looks like when you read it.
Much as there is an html standard, there are several differences in its implementation; formatting a mail message in HTML will only ever create an email that renders as you designed it in some fraction of mail clients. Microsoft are the traditional champions of standards deviation, and they also produce what appears to be the most populous mail reader. Creating an html email in whatever you use to write emails is therefore unlikely to render in mine in the same way, and neither needs to be particularly incorrect for this to happen.
This makes sense, though. Email has never been a means of sending beautiully typeset documents - it's for exchanging messages. If the formatting of a document is of particular importance, it should be stored in some format that always renders correctly, something like pdf or DjVu.
Plain text is substantially smaller than HTML. While I'm not on a limited mobile broadband contract, I'm still subject to the relentless advance of technology in the UK, and as such rarely come close to the 3mbps that T-Mobile promise me. HTML email takes longer to download than plain text, because it takes a whole bunch of characters to document the layout. Since I'm going to do my best to ignore these in any case, I'd rather not download them.
Plain text email doesn't feature images and obsfucated links - a URL in a plain text email can only possibly be a link to the url displayed, since there is no way of encoding it otherwise.
Finally, I like the convenience of being able to use a plain text mail reader. Wherever I am, if I want to read my mail I can log in to my mailserver, fire up mutt and read the mail. In plain text.