You're conflating brand with technology. Yes, the back end is the same, but the point is that the Bing-branded site is now ahead of the Yahoo!-branded site.
Bing gained market share at the expense of Yahoo. Google's market share remained flat for the year. The net effect is that Bing + Yahoo has not gained ground on Google. So back to my original question, what is the strategic significance of Bing overtaking Yahoo in market share?
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.