HTML mail is a mess that's impossible to get right; better to avoid it. Misleading users with a rich-text editor that looks like a word processor is bad because it'll pretty much never look as intended in anything but the same HTML editor that created it originally.
Annoying when pasting formatted text. Annoying when you try to delete a newline but it deletes an inline image. Sometimes tricky to add unformatted text after a formatted section (the editor assumes I want to continue the formatted section).