From a bargaining point of view, the move reveals that the majority values reaching 60 more than the minority values preventing it.  But this is a puzzle.  Why is it not zero sum?  A few reasons, some generic, some specific to this situation.

  1. In fact, other things equal, the minority should be able to muster more goodies because, due to the smaller numbers, each senator internalizes more of the cost of losing the fillibuster.  So there is even more of a puzzle.
  2. But the majority controls committee chairmanships which is a more efficient way to transfer value as opposed to bill-by-bill sweeteners.
  3. In the current climate the cost of losing the fillibuster is lower than usual because Republicans are lacking leadership and are generally adrift.  Their best chance to rise again is to give Democrats enough rope to hang themselves.
  4. As pointed out by Tyler Cowen, Specter already had some private motivation to switch.  He is among the most liberal of Republicans and his prospects for re-election are better as a Democrat than as a Republican given that he nearly lost the Republican Primary in 2004.