Re: Who relies on this stuff?
Err... it's a hack that can be used to decrypt encrypted content in some circumstances. If your content isn't encrypted in the first place, then it doesn't need decrypting, so this attack is unnecessary.
I'm a great believer in plain text emails, but I have to admit that HTML is also bloody useful. Mostly for tables - sending those in plain text is a right PITA, if only because the recipient probably isn't viewing a fixed-width font.
(Of course you can get around that by forcing a fixed-width font in your own message, using HTML formatting, so... what was I doing again?)