What the latest figures are on people viewing their emails in text only mode vs. HTML?
Wikipedia and it's source both seem to reference this research from 2006 which is an eternity ago in internet terms.
An issue with combining both HTML and text based emails is taking a disproportionate amount of time to resolve given the likely number of users it is affecting.
As with web browser usage statistics, it depends entirely on the audience.
I have access to a bit of data on this subject and it seems that text-only email use is very low (for non-technical audiences, at least). <0.1% up to ~6% depending on demographic.
It's not that much effort to do both (especially if you can find something to help you do the heavy lifting when creating multipart MIME containers), and you can always write a script to generate text from your HTML or something.