A text only email marketing campaign is not as effective as a campaign that uses HTML because it fails to compel and reinforce the intended message. It is not the same to send an email saying Summer Sale as one that has images that can put the customer's mind at ease. Even simple designs can have a significant impact on the look and feel of an email-message.
Using images, links, colors, borders, fonts and an effective layout reinforces the main idea of the campaign because the theme makes the message easier to grasp.
Remember you are trying to make a sell not break it. You must use everything you have on hand to make the best impact on your prospect customers.