[koralatov]

  > [...] but many people do prefer the extra formatting and
  > layout that a well-constructed HTML email can provide.

I think that's where the issue is --- nearly all HTML e-mail that I encounter is not of the well-constructed variety, and thus doesn't merit the extra overhead.

In a non-professional context (mailing lists, personal communications), I do just fine with plain-text; in a professional context, I loathe receiving e-mails that utilise what HTML brings because it's nearly always abused in the form of excessive signatures or massive legalese footers shrunk to 6pt, italicised, and rendered in nearly invisible grey.

Any content that really requires the extra formatting and layout offered by HTML would probably be better sent as an attachment anyway. I've lost count of the number of times I've had excerpts from spreadsheets or brochures pasted into an e-mail which have then been mangled in transit, or stretch way off the screen.