searches through Yahoo.com don't generate any revenue for Microsoft do they? Ie they show Yahoo ads, not ones that give Microsoft $$? I could be wrong about the deal specifics though.

Also, Microsoft doesn't really want people searching through Yahoo.com, in a few years time they could power their results through Google and Microsoft wouldn't be better off. They want brand recognition for Bing, so gaining market share for Bing itself is better for them.