Scott Lemieux further elaborates on the trouble with the Hillary Clinton experience argument:
[I]f Clinton's rather marginal and contestable experiential advantages over Obama should be decisive any of the other major Democratic candidates would be unquestionably preferable to either. (And, even worse, the same would be true of McCain in the general.)
I think the point in the parenthesis is key. If you win a primary on an "experience" argument, then you'd damn well better be more experienced than your general election opponent. McCain would make an experience argument against either opponent, so it's much better to be the opponent with a record of statements aimed at rebutting such arguments (I don't think the American people judge your qualification based on duration of service in a broken Washington system...) than to be the opponent who's been making the argument that voters need to stick with the more seasoned Washington hand.