I believe that blake8086 was referring to the content of the hidden message when he said plain text. He is asserting that if you hide 'hidden message here' in something, you are using security by obscurity, but if you try to hide ENC('hidden message here', 'secret key goes here') you are going to make the detection of the presence of your secret message easier.
If communication that you actually wanted to receive was sent in plain text then you could easily filter HTML email and be rid of it. Spammers aren't going to stop sending HTML mail because it is so useful for what they do, but no one else needs it.
If you're sending spam and marketing ROI is the goal, then you're part of the problem. Email was once useful for communicating with people but as the tide shifts from correspondence to commercialism it has completely lost its point. Nowadays an email address is little more than a way for people who got it to shove advertising down your throat.
Please don't send HTML email, there is almost never a scenario when it is necessary. All it does is create larger emails, have redundant information (most HTML emails also send a plain text counterpart), make reading email more difficult (especially if you are using a terminal mail client or are visually impaired), and allow for obnoxious email styling.
Taxes?