Ability to view how many times a recipient has opened email

Note it's merely a myth that anyone can accurately track every time an email was opened. (In any ESP/MA platform.)

And I'm not talking about people and mail clients that don't download images. Even when messages are fully image-enabled, mail providers -- and of course browsers themselves -- cache images, with vastly different cache lifetimes and invalidation criteria depending on the environment.

True, any tracking pixel download that manages to pass all the way through to the origin server represents an open. That is, it's not a false positive. But that's a fraction of the actual opens. I can reopen a message in Gmail 50 times and you'll see only one image download. If I refresh my browser, you'll see another hit, but those 2 hits are all the image server will ever see. Yet someone using Outlook might open the email 3x and all 3 may result in loading the pixel from the origin. So you'll get the mistaken impression that they were more interested.