I see folks at Yahoo beating their chests now that they've got some kind of pilot going on with Google, but how does Overture fit into this picture? They paid $1.6 billion for the company less than 5 years ago. What happened to that investment?
Originally, Overture's system was just rebranded as Yahoo Search Marketing and continued running as before. This system was then adapted and released as Yahoo Panama [google.com], their current PPC platform.
If your question was more about why Yahoo would devalue that by outsourcing to another PPC provider, I would suggest it's a business decision - Google seemingly makes much more per visitor than the other PPC engines, and so could be much more profitable for them. Whether or not this is a good idea long term is a matter of opinion.