Anecdotal, but I noticed a definite change in the tone of sales emails I was getting after I turned off images in Gmail. When they could tell I was reading their garbage but not responding, there was a lot of "tell me why you aren't interested" and "just a fifteen-minute call, just fifteen minutes we swear" and "please just talk to me, why won't you talk to me?"
They saw I was opening the emails—at this point, if I see something bold, I tap it without thinking—so they thought they had a chance, and they were really obnoxious.
I finally switched off images. After they stopped being able to tell if I was seeing their stuff, they got a little less pushy; they couldn't be sure I'd gotten their first email, so they focused more on re-explaining their product than trying to guilt me into reacting to them.
(I'm a little more understanding of people who use these to determine how much dead weight their mailing list has. Especially if you're an indie creator, you don't want to be paying for the 20% of people who might not be reading your newsletter. I don't mind a "please let us know if you're reading our newsletter" email as long as they're not obnoxious about it.)