I know how it works. I'm not interested in the ad if I'm not interested in the content, so the impression is invalid. And if the content is interesting, then I'll whitelist it and provide more impressions over however many years the site lives.

You lose the chance to earn those long-term impressions when you block people who use ad blockers. And you can run CPM ads on AdWords, so I'm assuming some AdSense ads are CPM-based.