Via a interview in B to B magazine [excerpt, my bold]: “It seems that Yahoo is out to get two main things from a deal with Google: a proven alternative to its failing search-monetization effort and access to more data that enable better behavioral targeting, complementing its technological and differentiating assets… A Yahoo-Google joint venture would produce the only entity on the Internet with access to a critical mass to enable true behavioral targeting. That’s a lot of private information in one place and a significant limitation for others to compete in behavioral targeting and personalization of the Web.”
source: “What a Google-Yahoo advertising deal means. Christopher Hosford. B to B. October 22, 2008