1. On one hand, the owner wants his visitors to stay on his site as long and as often as possible;
2. On the other, the site owner wants the visitors to "get out" as quickly as possible by clicking on the ads.
It's really hard to find out which of the above goals are more important for an average site owner. I guess - in most cases - webmasters prefer to have financial needs fulfilled first rather than doing the job for free.
That's why I look very carefully when I consider linking to a site with Adsense on them.