The definition of "text" itself is quite vague. Is a code sample text? What about a series of code samples intercalated with common language descriptions? What about a numbered list? What about a poem?

Also, just because I send some text as HTML doesn't prevent in any way the receiver from formatting that however they want. I'm just adding some display hints, that their email client may or may not ignore.

What about it? Your e-mail composer will tell you when you put something in your message which isn't plaintext and offer to convert your message to HTML for you to keep writing it.

You were claiming earlier that one shouldn't add formatting to their email, such as emphasizing using italics as it is the recipient who should decide how to format the email text. This is a completely different discussion from whether email that does happen to only require plain text should be sent as plain text or HTML.

I should have expressed myself better. If plaintext is adequate for an e-mail, then it should be plaintext. That's what I suggested. Of course, if you need to use other features, such as the italics you mention, then plaintext doesn't cut it.