Support: So because a small percentage of users use Email clients that don't support HTML well everyone should use plain text? By that thinking I guess they should stop rolling out hi-def TV as well. I mean, it costs a fortune in hardware to implement, uses way more bandwidth, way more disk space, and not everyone has a compatible TV. What's the point, right? Outlook and Outlook Express dominate the desktop Email client market in home and corporate use and both support HTML just fine. I can already hear the response to that - "not everyone bows down to Microsoft...". Maybe true, but a vast majority of home and corporate desktops are Microsoft based and they work just fine with HTML.
Security: As far as security, everyone should be running a virus scanner these days. Most Email viruses are triggered by stupid users launching attachments anyway. Plain text isn't going to prevent that.
Size: If you're sending graphics in a message why would it make a difference if it's embedded or attached? Either way the graphic is going to be there so size isn't really an issue. I don't put images (screenshots for example) in the body of my message but at least the option is there if you want to.
Appearance: Plain text Email is nothing special, maybe more standardized/accepted but that doesn't mean it looks good. To me it's just limiting - you can't do things like bold, italic, underline, change color, etc in plain. HTML Email can do all of that and more. Maybe some don't feel those options are necessary in Email communication but I like having the option. It's quicker and easier to do that in short Emails than having to create a document and attach it.