Used to be, people came looking for a tool like MailChimp because they wanted a way to embed a plain-text alternative message along with their HTML emails. Back then, email programs were unreliable at displaying HTML emails. People wanted a "backup" email to display for recipients who used old email programs.
But now that so many email applications render HTML email just fine, marketers seem to have forgotten all about plain-text. More and more, when I look through recent campaigns sent from our system, our users seem to be skipping the plain-text step altogether when they create their campaigns.
This is a bad idea. For one, spam filters don’t like it when you only send HTML email (lazy spammers do that). Two, people who check email on mobile devices almost always have problems with HTML email. We’re not even going to get into all the various ways mobile devices can mangle HTML. The safest thing you can do is give your members the choice to receive the plain-text version of your messages, and always create a plain-text version for each campaign you send.