Re: Yet ANOTHER reason!
Add in CSS and media queries and you've got a nice little device sniffer that tells the sender exactly what capabilities your device has
All you need is an MUA that renders external images and you have webbugs. You don't even need CSS for that one. (Or an MUA that respects the onerror event, though I'd hope that one which doesn't render external images also doesn't follow onerror.)
Of course, MUAs that respect CSS often provide other webbug channels, such as background-image.
And HTML emails make phishing a lot easier.