Why would Obama choose - as is being widely rumored - Senator Joe Biden as his running mate? Foreign policy credentials, sure. But there are plenty of smart, and younger, Democrats who could bring credibility and experience without bringing Biden's loose lips. Biden will inevitably cause embarrassment on the trail and in office; like John McCain, his brain/mouth filter has some big gaps.
But Biden will be 73 in 8 years, too old in all likelihood to run for president. Hillary Clinton will be a crucial four years younger. So maybe a Biden vice-presidency is a compromise with the Clinton branch of the party: they understand why Clinton simply can't be Obama's VP, but they can reasonably demand a choice who won't unseat Clinton as the "elder statesman" of the party when Obama leaves.