What, is he going to become a Republican again? What could they possibly offer him unless the midterm is an unforeseeable bloodbath? Why would they accept him back, in the middle of their ideological cleansing?

Before Specter made the switch, he offered to help with designing amendments to the National Labor Relations Act as a compromise for getting some of the Employee Free Choice Act's provisions past the perfunctory minority resistance. He also pays attention to what the Supreme Court does, and has introduced legislation to overturn some of their recent questionable decisions (e.g., screwing with a decades-old liberal pleading standard for plaintiffs in Ashcroft v. Iqbal, with no legislative or constitutional basis).

I think Sestak is a lot less important than Reid and his whips, quietly doing their jobs - if only because Sestak's candidacy is too recent for several of his changes of rhetoric. If we treat Specter well, I suspect we'll keep his vote on many more issues than we'll lose him.