Allowing the president to pick and choose among budget items is admitting that Congress is too irresponsible to fulfill its own duties, and needs a grown-up monitor. We can’t stop spending, so we’re going to ask the president to stop us!
Just ask the Minnesota state legislature how much they like giving the executive power over the purse. I'm not sure what makes the borderline psychopaths that run for executive offices more fiscally responsible than the borderline psychopaths that run for legislature. For some reason, Mark Kirk has faith in the executive branch.