Lack of ability to distinguish between pre-formatted text and regular prose alone makes it a complete non-starter for most who aren't reading monospaced text in a terminal. I don't really like reading monospace text for prose (many people don't), and using proportional fonts means things that are supposed to be aligned will break (and if you restrict yourself to text only, you'll find yourself aligning things on occasion for tables or other content).

I actually use plain text in FastMail because it's "better" than HTML (usually), but it's not good.

Ability to send text/markdown natively would be pretty brilliant.