In most cases having a great email design is really important.
Sometimes though a plain-text style* email can actually perform better. If you have a very specific audience and a short, highly targeted message with a clear call to action then a simple email which focusses the reader on the message (rather than the branding) and emphasises the call to action can have dramatic results.
In the example below (promoting our 2014 email marketing benchmark report the message text was identical in both emails, but by focussing the reader’s attention on the message the plain version actually generated 2.5 times as many clicks.
Plain-text style emails are also more likely to land in the Priority Inbox in Gmail, which can further improve your results.
*Plain-text style, rather than actual plain text, as they still use HTML (so that you can use text styling, and include tracking), but visually they look like a standard email written in a normal email client.