You have it backwards. The more a publisher knows about it's users the more targeted ads they can sell for higher rates. Compare CPM on myspace.com (pennies) vs the wsj.com (tens of dollars). Yahoo and Google have similar people searching, but Google has been able to provide more personalized ads and therefore make more money. Advertisers measure their return for every dollar spent. If Google doesn't keep on doing a better job than other sites at personalizing pages their users will go elsewhere where they can find what they need more quickly.