Page and Brin themselves once pointed out the problems of accepting ads or paid placement, with some rather ironic examples: Furthermore, advertising income often provides an incentive to provide poor quality search results. For example, we noticed a major search engine would not return a large airline's homepage when the airline's name was given as a query. It so happened that the airline had placed an expensive ad,…
>If people searching for Southwest Airlines on Google aren't ending up on the Southwest Airlines website without a huge great banner ad (despite it being ranked at the top of the results) then something is going badly wrong on the Google search results page. Or maybe they are, and this is simply an example of a branding bonus/partnership for big spenders. We have no idea what the economics are and why.
What branding benefit can there be in showing a text ad to a user who is already searching for your website?