I'm sure that you don't have a problem if confirmations are unstyled. But I'm also sure that a lot of users differ in that regard and would expect at least some

I'm sure that you don't have a problem if confirmations are unstyled. But I'm also sure that a lot of users differ in that regard and would expect at least some styling on that non-marketing, but still system-generated emails. Examples include things like confirmations, notifications (that you've opted in to), or just richer and more complex conversations. I mean: to end users, the email experience is part of the brand, and if the brand emits terribad emails, or unstyled emails, it erodes confidence. I'll grant that there's a lot of yucky and annoying html in some marketing emails. But there is still a use-case for html-styled emails, and just having plain text with "click here to see images of the food you're getting next week" is pretty darn 1994.