There are noises out there on the blogoblags—noises with which I have some sympathy—that Arlen Specter's party switch has made it difficult or impossible for a mainline Democrat to represent Pennsylvania, and that he ought to face a robust primary challenge. At present, Specter enjoys high approval ratings from Democrats, but it's unclear how long that will hold.
I think it's worth considering that, should Specter face Pat Toomey in the general, Democrats will not have to spend quite as much money in Pennsylvania as they will if Joe Torsella is the candidate. Or Joe Sestak. That has value, since it means the DSCC and major national donors can move money elsewhere. Less money for Arlen Specter means more money for Paul Hodes in New Hampshire, or Chris Dodd in Connecticut, or whatever challengers emerge to Richard Burr, David Vitter, and the curiously unfunded Tom Coburn. In otherwords, having Specter as a "More Democrat" may mean that elsewhere Dems can fund a "Better Democrat" to make Specter's vote that much less essential.
I don't think this is a open-and-shut-cut case. I think there are real reasons to be frustrated at the Pennsylvania machine's decision to try to clear the field. I also think that the threat of a robust primary challenge will help ensure Specter maintains a progressive voting record (though I note that the notoriously pro-choice-except-when-it-matters Specter voted for the Sebelius confirmation). But Philadelphia is a huge media market, and Pittsburgh isn't exactly small. It's a lot of dough that could go to other states.