Partially-relevant search results, identified as "containing multiple concepts, [or] on target but too narrow," play an important role in a user's information seeking process and problem definition.So, the paper suggests that there may be multiple benefits to personalized search. Not only do we get the benefits of improved understanding of query intent and increased relevance, but also we can improve diversity and discovery.
By studying Web search query logs and the results people judge relevant and interesting, we find many of the queries people perform return interesting (potentially serendipitous) results that are not directly relevant .... More than a fifth of all search results were judged interesting but not highly relevant to the search task.
Serendipity was more likely to occur in diverse result sets .... Personalization scores correlate with both relevance and also with interestingness, suggesting that information about personal interests and behaviour may be used to support serendipity.
For a discussion of yet another benefit, reducing the payoff to web spammers, please see also my July 2006 post, "Combating web spam with personalization".