Inline images are very useful for diagrams, sharing screenshots and more. I can do without the stupid company logo underneath every email, but I'd much rather have to put up with those than be unable to use them. The resources need to be small enough to embed inside the email, though, and external resources should be made aa difficult as possible (always hidden by default, per rfc, with at least two clicks before they're displayed).

My perfect world email markdown will probably be different from yours. For productivity, anything more than slightly stylised text is just unnecessary, but in practice, most email isn't used for productivity anymore. Instant messaging has replaced email as a means of conversation in all workspaces I know.