The Yahoo thinking may be that given a world where demand-side platform technology can bid across inventory sources, why not make Yahoo inventory biddable through Google, where the tech may be stronger let alone the aggregated demand? Google's demand-side platform DoubleClick Bid Manager ("Invite Media 2.0") is the largest buyer in the exchange tech business today.
Such an outsourced approach would be consistent with Marissa's early focus on content and products, at the expense of ad tech. The long term could be viewed as a way to hand ad tech off to someone who can do better. Put more simply: Yahoo may not want to invest any more money in RMX.
On the other hand, if this is about leveraging contextual, Google's contextual wins out given the demand they own. The deal makes sense even without all the above speculation.
From the Yahoo post:
"Today, we’re excited to announce that we recently signed a global, non-exclusive agreement with Google to display ads on various Yahoo! properties and certain co-branded sites using Google’s AdSense for Content and Google’s AdMob services.
By adding Google to our list of world-class contextual ads partners, we’ll be able to expand our network, which means we can serve users with ads that are even more meaningful.
For our users, there won’t be a noticeable difference in how or where ads appear. More options simply mean greater flexibility. We look forward to working with all of our contextual ads partners to ensure we’re delivering the right ad to the right user at the right time."