As I've said before, there are searches for consumption needs, and a completely different kind of search made for research needs.
No popular search engine today differentiates between the two, but Goggle is slowly moving towards a search dedicated to consumption needs, which seems logical for an advertizing company.
I might search for pizza in order to find a place to order it from. Or I might become curious, and search for pizza to learn about its origins and history. A personalized search would only answer the first option.
Personalized searches also render it difficult to gauge how 'pizza' relates to other data in the public (sub)conscious. Google results used to (and still do, to a certain extent) provide a very good picture of the semantic structure of the Internet, if you knew how to read them.
Having a mental model of how the google search ranking works sounds… hard. For me the mental model works out to be "based on various statistics, google's best match for my search terms". I'm not sure that's different from "the best answer". :-)