You and I are in the minority. Imagine if Groupon sent their daily flyer in plain text: Click-through would plummet.
Maybe it is just that most people have never been exposed to properly formatted text emails?
I'm sure people find most of the websites they visit and the emails they read, filled with columns of extra garbage, to be cumbersome and difficult to read. Executives think that's what people want, but I doubt there has ever been an thorough examination of all the formats and their effects on people.
Comment nesting must have been awful. Or did you cheat and use pseudo-graphical characters to overcome the obvious limitation of plain-text for such a purpose?