HTML messages are nice and simple, so as a marketer using emails you want to use them. The customer may not appreciate these emails as much. In fact they might not open them. Put your customer in control by offering a text version that they can subscribe to over the HTML version. Also, you can send these emails as multi-part meaning they have a choice right in the email. In this way there is only one list and version you have to send out. They get to choose what they wish to see. Giving a choice is nice since it puts the consumer in control.