That's a load-bearing "if present" there. Companies that want to track you can just send email without plaintext versions and you're still vulnerable.
How? Even if they write in HTML only there is nothing which could prevents a email client from just not rendering them which would mean plaintext.
And you'll probably get a lot of empty emails you can't read that way, unless you configure the plaintext plugin to dump the HTML into the reader view.
Not at all. You will see the HTML code but not render which is easily readable since HTML is very simple.