and if you use a reasonable email client, like Thunderbird, you can choose to block all HTML/Images which communicate with an outside server.
Yes. That's what I use, and it does block the service. Outlook 2003 (which I use at work) also has this feature.
Even more to the point it doesn't really prove much.
Yeah it proves that the email reached the recipient, and I guess that matters to some people. But it doesn't tell you that the recipient READ the email, only that he/she opened it. Since many mail readers (eg Apple Mail and Outlook) open mail pretty much inevitably (eg, when you select junk mail to delete it, after you hit delete the next mail down will be selected and "opened", the act of opening an email message has only the slightest correlation with the act of reading it.
your argument presupposes cluefulness on the vast number of email users out there.
HTML doesn't belong in email anyway. Wouldn't work by default with mutt, pine or emacs.