When someone sends me an e-mail, I want to read the message they've sent. I don't want their idiotic decision to render it in unreadable tiny blue lettering to factor in to this. I don't need a half-megabyte PNG showing me what their corporate logo looks like.

I don't want my mail client to have to rat me out to the sender when I open the message, just because their template depends on loading images from the web (apparently some mail clients now do this again by default? Madness!).

I don't want to parse the sender's message through a complex (and historically fragile) HTML parser just to give them the opportunity to make the message more difficult to read.

E-mail is about communicating a textual idea - the text is what matters. Let me decide how I want to render that so I can best digest it.