People need very large red "click here" call-to-action buttons to click inside emails. With plain text email you can't have those. Also people need a sampling of your latest blog posts in a table inside all your emails, with a stock photo in the left column and the article title and snippet in the right column.
Prior to computers most business correspondence would be typewritten in essentially the same font with little or no graphics, maybe a logo on the letterhead if you’re really fancy. That seemed to work fine.