I think what Joe Klein writes here, Krugman Quibble | Swampland, is exactly right:
If he had cited the 14th Amendment and simply ordered Treasury to pay the bills, he would have been impeached by the radical Republicans. This would have guaranteed that the next 16 months would have been overwhelmed by an even worse version of the silliness visited upon our nation by the poisonous Limbaugh-Tea Party nihilists.
If anyone thinks differently, I would like to hear the argument. You already have people like Issa who could have kicked this off in a heartbeat. And based on the arguments I saw for supporting the use of the 14th Amendment, I concluded that no one had a clue how it would actually work out.
Whatever Liberals think of the deal, that option would have been far worse.