And for sure, you do make some good points on where HTML email can be a good thing. Perhaps I'm being too pessimistic.
Much much worse, however, are all of the people/companies that include multipart/alternative sections, but do it completely wrong: "This is an HTML-email. Your client lacks HTML support. Please visit [url here] to view this email" or even worse "This is an HTML-email. Your client lacks HTML support. Please upgrade your software".
I come across these almost daily, and it bugs the hell out of me. Unless it is something very important (bill, sign-up email, etc.) I simply ignore these emails, and they never get read. (Yes, it really is out of spite. I'll ignore them when reading email in a web client or on my phone later. They simply lost their chance)
I don't really like HTML mail, but I can live with it. I see the reason why it exists (great for advertisement). However, if you use HTML email, do include multipart/alternative section for non-HTML clients, and include the same information there, non of this "your client can't handle HTML, you loose crap".