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I think MSN is useful in that it provides Yahoo SERP's, which are very good. For people who are used to using MSN, who have it as a default on their Windows-based computer, and who don't know much about the web, these results are useful because they wouldn't know to search elsewhere.
Yahoo's definitely a force to reckon with now as their traffic is more qualified than Google's and they now power a huge part of the web.
Now whether that contract is null and void after the purchases remains to be seen. If MSN still is bound by that contract, that would explain why they want to be independent but this summer.