For those that don't understand how browsers work; MS Bing presently has a tremendous disadvantage by not having the same database of user searches to refine their search engine's formulas. Virtually every user search, whether it be for Michael Jackson, Kate Spade bags, airline reservations to Rio, is research data to improve the engine. Things like misspellings, syntax and the user's response to the search results are compiled and adopted into the system.
Bing has some catching up to do and it's counting on Yahoo! users to help