Matt,
When sending HTML messages you must also include a text message. Messages are sent in "multi-part" format, which is text plus HTML all in one, with the subscriber's email client deciding which version to show. If you were not to include a text copy of the message, and the subscriber's client doesn't support HTML, the user would see a blank email.
One alternative to creating a completely separate text copy of your message is to use a message code of the type "Link to HTML" in the text section of the message.
Regards