How else would you suggest designing an email? Plain text? Clients aren't just going to be like..."yeah, we don't want any images, or buttons, or fancy designs"...
Whether you're consciously thinking about design decisions or not everything is designed.
The better question would be, why does design matter?
This list could be huge but here are a few common reasons:
- To simplify information.
- To make it more digestible.
- To be engaging. To invoke emotion.
If you blast an average email receiver with a poorly designed wall of text, at best, they might read part of it and drop out half way through. Most of them will archive or delete it without even opening it and another portion will probably report it as spam.
I won't sit here and discuss the complexities of why design matters but here are a few links:
People said the same thing about web design too. "Why can't you make your site simple enough to look good in lynx?"
It's a legitimate point.
However the main thing you should realize is that the world doesn't care. You are not paying the bills for these companies, and thus your opinion is moot. HTML email is popular because it works. No amount of nerd rage will make a dent in that. Don't let that stop you from using mutt and aggressively spam flagging anything without a plain text part, but you might save yourself some embarrassment by not farting into the wind over it.
I swear all the devs on HN work in product.