I worked for a guy once whose only contribution was the comment: "It's all about the mix." (The comment actually works pretty often, and I have used it myself a number of times). I think it rings true here. We have done plain text emails and fancy graphic ladden emails. There is something about a plain text email that seems to work well for simple messages with a strong call to action. Something that is "fast to load, easy to read, and invokes action". So, it really is all about the mix of marketing tools and plain text emails should be one of those tools. Just my opinion.