How would you feel about your customers sending tracking images to you with orders/complaints/queries? Just to "fine-tune" whether they deal with you again? I imagine it could be statistically enlightening to see how quickly you open emails, how often, and how long the response takes. Not so keen?
I appreciate your efforts to ensure that your emails lists are on target and not spammy, many companies are not so diligent. (Particularly with confirmed opt-ins.) But you have no automatic right to collate any further information about your customers unless they intentionally provide it. Tracking images are sneaky and most certainly not used by your customers intentionally. There is a reasonable expectation of privacy when reading your own email on your own computer.
You're right about two things though. The days are long gone when spammers cared about whether an address was valid or not. They are not incurring any costs spamming to invalid addresses. All they care about is how many suckers they hook with a response. And yes, the cached image hits are yet more information being sucked up by google, that will inevitably be sold in some way in the future.