It's HN, so I'm not surprised by the old "just send text" argument that always gets wheeled out when this topic come up, but for those not in a terminal all day there are valid reasons to format HTML emails (that don't include marketing).

We send out HTML emails and reports to our users that make use of progress bars, lists, photos, colours etc and they love them. They can get quick full updates on their business without having to leave their mailbox. Pretty hard to do that in plain text.