For the longest time (and I mean a LONG time, been sending email since the 80s), I absolutely despised anything other than plain ole ASCII text for email.
I would actively seek out the settings in whatever email program I had and make it so that it wouldn't send out anything other than ASCII. No fancy crap whatsoever. Z-mail, Netscape Mail, Outlook, you name it. I'd also go hunt down people who would attach big files (100kb! LOL) to emails and tell them to go put it on a server and to send the link to the server instead.
But times have changed. We've got better standards now, and networking and compute horsepower are basically off the rails compared to back then.
Somehow, though, even though Apple started the WYSIWYG revolution, Mail.app carries at least some of the same types of thought patterns I once did, and there are really simple things that other email apps do that Mail.app doesn't.
I still tend towards simplicity with email, don't get me wrong. But in the corporate world, having (and establishing) a brand is important for a lot of people and email is a big way of doing that. All we're talking about is having the equivalent of letterhead for email, and again it's curious as to why Mail.app doesn't support that, Apple being who they are.