Can you "defect" from a party that's made clear its intention to have you declared a non-person?
Arlen Specter was almost certainly going to lose the Republican primary to the Right Wing Club For Growth darling Pat Toomey and if and when that happened Republicans would have cheered.
Democrats would have cheered too. Toomey will be relatively easy to knock off in the general election and if Specter had opted against a probably doomed run as an Independent Pennsylvania could have had another real Democratic senator.
Specter's switch (or his return to his roots. He used to be a Democrat, a hundred years ago.) turns the general election into an open Republican primary.
The advantage for Democrats in having Specter on their side of the aisle is that Specter the Democrat will probably not be as in favor of filibusters as Specter the Republican was. Also Specter the Democrat might remember that Specter the Republican used to be pro-Union and a supporter of the EFCA and vote for it for old time's sake.
Who knows? Specter says the Republicans have become too Right Wing for the likes of him, which is a truth you'd have thought he'd have recognized, oh, about 12 years ago. Obviously, he doesn't think the Democrats are too liberal for him, and why should he, since he's taking his place in the Party to the left of Ben Nelson and Kent Conrad. I'm not sure how much of a Blue Dog Specter's likely to be. I don't expect him to be a reliable liberal vote, but I expect him to be unreliable in more of an oddball way, more like Joe Lieberman than Evan Bayh and Blanche Lincoln. Many progressive bloggers have expressed varying degrees of dismay at having Specter join the Democrats. The consensus seems to be that Specter's groveling and crawling to the Right over the last eight years is more revealing of his true political nature than the moderate to liberal (for a Republican) stands he took during the twenty years he was in the Senate before George Bush became President. People remember his appalling behavior during the Clarence Thomas hearings, but not his votes against conviction during the Impeachment Crisis. The rap on Specter that his true political nature is not conservative, moderate, liberal, or idiosyncratic, merely opportunistic. Arlen's always been in it for Arlen. But if that's the case, then it's in Arlen's best interest to be as a good a Democrat as he's promised the President he's going to be, for at least the next year and a half.
The Republicans know this and this is why they're mad. They wanted him where they had him, groveling and crawling for the next year and a half, at which point they were going to bid him adios. There's probably a pang on their part that they've been denied the pleasure of shit-canning him themselves. They're like the boss who's left standing spluttering in his office after the employee he planned to fire has announced he's quitting to take a better job with a rival company.