My AdSense rep encouraged me to set the "Sites authorized to show ads" because she has seen accounts disabled from 3rd party websites using AdSense on spam sites with the purpose of getting the AdSense account shut down. I did so and here are my observations from the first month.
There are now 84 sites listed that tried to show my ads but which couldn't. Most of them are cache sites and Google translate on various top level domains.
A few cache sites that are trying to show my ads block Googlebot via robots.txt. These sites had been causing "Robot Denied" crawler errors. I was hoping that Google would stop reporting these errors for sites that I don't claim to own. However, these sites still show up in the crawler error report.
My estimated earnings takeback was lower this month. This month my finalized earnings were 99.2% of my estimated earnings. This is better than it has been. I'm thinking that some of the estimated earnings that don't make it to finalized may be ads that show up on other sites.
...estimated earnings that don't make it to finalized may be ads that show up on other sites.
It's a lot more than that, but maybe a slight factor. I have always specified the sites (my site and YouTube) that can show my adsense ads. I even exclude Google translate and google cache because I believe it brings down the CPC. Less impressions? Yes. Higher CPC? Usually. Most of the sites I've seen that try to display my ads are scrapper sites that copy the entire HTML code inside the body tags amd obviously have no regard for anything.
My site does not prohibit archiving with the robots tag. Here is the breakdown:
64 translate.google.* 7 www.google.* webcache.googleusercontent.com websearch.cs.com search.yahoo.com www.bing.com then about 10 other sites, some of which are certainly scraper sites, but some claim to be cache and some claim to be proxy.