If you send a plaintext email that is more than just a link to a dodgy domain you are guaranteed to get through (unless you're already on a blacklist). This is the only kind of spam I still get. Even spammers know to use plaintext (well, some)!
Well, yes, mostly. But you also want your email to be read, not only delivered. Your email mail also contain some kind of CTA (call-to-action, website to visit and so on...) which you are trying to get the reader to respond to. Those numbers may be higher with well designed "beautiful" html mails than plain text ones.
Lastly, you may want to measure how many people actually opens and reads your mail and when they do it, instead of just letting it languish in your inbox. It's trivial to do by embedding tracking pixels in the html mail, but impossible in plain text ones. Personally, I abhor html mails like the plague, and the people who believe that bold facing or underlining some text somehow makes it easier to decipher some meaning from the text, but they do have a purpose. Mostly for marketing purposes I admit.
