People who use screen readers would probably be very supportive of plain text Web sites. Web devs usually do a lot of work to make a Web site accessible.

In any event, email and Web sites are completely different things. Different communication mediums require different communication styles. Advocating for a certain style on one does not mean you have to also advocate for the same on another. The question would be "does rich-formatted text substantially add to the medium of email," and I think the clear answer to that is "not really."

An example of a good use of rich-formatted text contributing to an email would be coloring responses to questions. I see this one a lot, e.g. "my responses are in RED." We had the > quote style for this already, which was pretty good until an email thread started to get really, really deeply nested. In plain-text, at some point, somebody would have to edit the quotes down. There was a marginal advantage to having styled text to clear this up. Was this advantage enough to have to suffer through the majority of emails where this was not required and the formatted text was rife with extraneous or useless widgets?