I have yet to work out why anyone should wish to turn on html in emails ever and I always thought that was the start of email being broken. Protocols existed before machines and should serve us as well as them.
People turn on HTML in email because there is no other widely accepted alternative for rich text email.
As for why plain text only is not good enough, here's a simple thought experiment. Imagine that we did not have any kind of email. We invented computers, and computer networks, but somehow, incredibly, overlooked inventing email.
In this hypothetical world, people still communicate by writing letters, and sending them through the post office. Of course they write the letters on computers in word processors, and then print them, and it is the print outs that they mail.
Now, imagine in this hypothetical world that someone finally comes up with the idea of email, and pitches it as an electronic equivalent of regular mail that is faster and more convenient. Is he going to make it plain text only? Of course not. It will need to be as capable as physical mail, which means it needs to support sending anything that people can print on paper. That means some kind of rich format that can handle different typefaces and fonts, colors, inline images, and attached documents.
Email started off plain text only simply because when it was invented the technology wasn't up to the challenge of handling the presentation features of real mail. The technology has improved, and rich email is the natural, inevitable outcome.
So, by the same logic, twitter should allow javascript to execute when I view some post about people microwaving small mammals.
Perhaps I should be more explicit.
My problem is not with the abilities to assign pixels to an output device. Email should not be an interactive proposition and should have no ability whatsoever to run code.
