I agree with the suggestion that you analyze how much of your sales are to customers who prefer plain text and assess whether it covers the cost of maintaining it. However, the numbers might not tell the whole story because some people deliberately block or ignore html mail even though they could figure out how to read it if they really wanted to. You probably don't monitor the replies to your marketing emails so you'll miss the replies requesting plain text format. (Nobody's going to look through your html code in their plain text mail reader to look for the url they have to copy and paste into a browser to access your web app that lets them change their email preferences to read your future marketing emails. They probably consider their time at least as valuable as yours.)