Like it or not, any politician running on a party ticket owes a duty to complete his or her term as a member of that party. A duty to the party and to the voters. If he or she believes that they can no longer represent the voters through the party, then they should resign first, then join another party, and then stand for reelection.
The fact is, like most national politicians, Specter stands for the only principle he knows: Getting reelected.
Update: David Broder on Specter the Defector:
The one consistency in the history of Arlen Specter has been his willingness to do whatever will best protect and advance the career of Arlen Specter.