The problem is that "show this E-mail in text only" relies on the fact that whatever they use send out the E-mail also sends plain text as well as HTML as a multipart. I find most of places do that, but there are some services/implementations that simply sends out HTML, pretty much leaving it up to the mailer to interpret that E-mail.

Either you will be sending in HTML or plain text, I feel there shouldn't be any excuse of not attaching plain text version of it.