That's interesting, mine definitely only has plain text (no text/html, checked via view source etc). Maybe it's a preference I ticked somewhere back in the mists of time.

I also used to tick that preference when a choice was provided, but have stopped doing so after encountering too many cases of text/plain versions being broken one way or another. Not that common, but still too much of a hassle.

Sometimes they are "empty" like here, or contain placeholders in place of the actual data, or just break random features (e.g. eBay prevented other users from sending me photos).