I just don't understand why any mail client in use today would not support HTML; or why people would turn it off? Surely, unless you're using OE, security isn't a problem. People seem to manage browsing the web without issues, and that's a far less controlled and predictable environment than email.
I get sick of people sending me email with colours that make my eyes bleed. I hate people who send me email in random fonts that they've found on the internet that make me guess what they are trying to say. I have grown a healthy feeling of rage at people who think that I really wanted to download that 600mb image of themselves for their sig. I like my mail to be nice and consistant. I have eyesight issues, so I pump the fontsize up on my mail, and late at night when I'm tired I just can't be bothered squinting at the screen to make out what someones written who decided that I really needed to see it in their favourite 3pt font size. IT may be easy for /them/ to read but I can't. I force my mailer to display as text/plain always. Thus I get a consistant, easy to read mail that I can see, people trying to convince me to read mail their way just end up in the bitbucket. As an aside, spam often doesn't have sane text/plain parts, so I skip on all the weird porn spam too.