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).
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).