Aside from the fact that a weakened but triumphant Specter is much more likely to lose the general election, and that a victorious Toomey is almost certainly doomed, that is...
It's that party-line voting just makes sense as far as congress is concerned. Congressmen are one out of 435 and Senators are one out of 100 (theoretically). Their individual actions aren't likely to change things too much, unless they're really talented. So, picking someone who shares your party label makes sense, especially in the Senate where procedural votes are the big deal, and those votes are almost always party-line. Hence the strange spectacle last session of Chuck Hagel voting to filibuster his own Iraq bill. It's tradition, I suppose.
So, it really doesn't make too much sense, and that the GOP can't afford to shed too many more high-ranking officials evidently factors into this calculus. Specter opponents will no doubt insist that he's little better than a liberal Democrat, which is not really true, as that would mean that he'd vote with the Democrats on procedural notions with almost uniform frequency.
Update: Wouldn't Specter's relative moderation be a huge asset for him? He's one of few "gettable" Republicans and therefore has vast influence over what actually gets passed. Seems that would be a compelling pitch to me, if he survives. It makes little sense for the guy to stay in the GOP, since it isn't going to help him and will, in fact, do what it can to run him out. But I don't suppose he's the sort to switch parties.
