Other arguments against it include what I call the 12th Amendment argument (it's dangerous and unproductive to have political enemies in the executive's top 2 jobs), and the way it would tie Obama's hands. It's often nice to look like you are working across lines on national problems. But often that means a constraint on the president's ability to implement his program. And no matter how obstructionist or difficult or damaging a Vice President is, the president can't fire him or her. This is a perch from which the Clintonistas could keep fighting for their way, no matter what the occupant of the Oval Office wants. And, therefore, it's reasonable to think he might want to keep his enemies and competitors out of such a high profile post in his own government.