Is Arlen Specter a craven opportunist, a Republican Joe Lieberman, jumping parties so he can avoid a humiliating primary defeat and hang onto a cozy government job for 6 more years?  Sure, but come on, he's a politician. 

Is he going to vote with Republicans most of the time?  My guess is, more than we'd like but less than some expect.  He doesn't have the Repug leadership bullying him any more, he needs support from the Dem leaders and he does sincerely want to be part of governing, not just obstructing.  I think it's more likely that he will forge partnerships with a bunch of other center-righters (mostly Dems at this point but Snowe & Collins could join in) and insist on watering down a lot of key legislation in exchange for supporting cloture.

Why not just let him hang in the breeze and run a more liberal Dem for the seat?  Because we might not win.  This is a sure thing and it would be reckless not to take advantage of it. 

Is it bad for the Democratic party that there are so many center-right Dems nowadays?  Well, it's better than having these guys be Republicans, isn't it?  There is always going to have to be negotiation and compromise between the center-right and the center-left to get things done (the less centrist folks are going to be less inclined to compromise by nature).  So now instead of compromise happening between the parties, it happens within a supermajority party.

The Senate by design underrepresents the densely populated urban centers of our country - this tends to put it to the right of center on most issues.  We are better off with a strong center-right block inside the Dem caucus that wants to be an active part of governing, than with a larger Republican caucus focused only on saying no.  So let's celebrate this as a win, which it certainly is, and welcome Specter in spite of policy disagreements and general distaste for such obviously self-serving opportunism!