I'm not sure. It would appear, to me anyway, that this is a relationship between Google and the end-user - not something the site owner has any control over, outside of referring members to the service and/or encouraging them to contribute.
It's currently only for U.S. residents too so not much value in it for me just yet.
This is controlled via Google. The more a person contributes, the less times they see Adsense ads on your site. The ad count to contribute ratio is based on the value of your ad and the user must contribute to your site. In a nut shell, the more they contribute, the less times they view Adsense codes to display ads.
I do not think it would be useful because it takes away from us in a way to offer premium memberships for less ads. Adsense does not pay that great if you a a small forum with good traffic so I would rather the members pay me direct then waiting on google.