“Why would she choose him?”
There’s only 1 reason - to unify the party against McCain. It’s their best chance to beat McCain.
However, Obama will never stand a chance at the presidency in 2012, if he settles for VP now. Because Hitlery is not going to make people happy and she’s going to drag her VP down with her. If he waits and let’s Hitlery screw up and then runs in 2012, he’ll have gained more ‘experience’ in the eyes of voters, and stand a much better chance of being the nominee then.
Hillary, if given a choice, will chose a VP who will best help her achieve her life-long ambition, IMO.