This is in part a cultural thing. Within GNU, many are hostile toward non-plaintext e-mail and might even ask you to resend plaintext. But we're a group of people that live and die by text. Many hackers are extremely efficient in processing text. Adding document formats like HTML into the mix muddies things unless you have very specialized tools for dealing with it.

And then you have people who start using formatting semantically, e.g. colors.

Now, with all of that said, Emacs does a remarkable job of rendering a subset of HTML. For example, I have my mail client configured to always ignore HTML alternatives unless they're the only option. In that case, colors and tables and such are all rendered as they should be. Inline images might be supported if you use a graphical version of Emacs (Emacs can definitely display images, I just haven't tried in this context); you also have the option to open it in an external viewer, like a web browser.

I think emacs can probably do a reasonable job of processing and displaying the subset of html that is often used in email. For anything really fancy you can open it in a browser (which is something I regularly do in outlook as well).

He's not getting rich emails, he's only getting patches. Works for him. Wouldn't work for me.

He doesn't seem to get many patches. Most email discussions where I see him involved are, well, discussions. This rarely involves patches.

The only reason I still open Outlook regularly is to deal with calendar invites. Otherwise, gnus actually works just fine for email. Better, in many ways.