I like Publius' point that the Specter switch may have been a happy result of Hillary Clinton keeping the primary going several months after astute observers knew she was going to lose. The desire to vote in the ongoing Democratic primary gave people an incentive to register, and the primary itself gave the Obama and Clinton campaigns a reason to register every voter they could within each of their preferred demographics. These processes turned 200,000 Republicans into Democrats in 2008, and they've turned at least one Republican into a Democrat in 2009.
There's an interesting theoretical issue here about how large you want your party to be in a state like Pennsylvania with closed primaries. When your party gets bigger, you're more likely to pick a winning candidate, even if you get bigger in a way that doesn't involve anybody agreeing with you more than they did before. Assume for the moment that none of the 200,000 people who left the Pennsylvania GOP left because their ideological views had moved leftward, and they just switched for the fun of voting in the Democratic primary. Also assume that their voting behavior in a general election stays exactly the same as they were when they were Republicans, so they're just as likely to vote for Specter in a general election as they were before.
Even under these assumptions where we don't convince anybody of anything, the Republican Party's chances of winning general elections decline. A smaller party is less likely to match the sentiments of the general Pennsylvania electorate -- it'll just be a bunch of people with idiosyncratic views. Candidates selected by the Growths who remain in the Pennsylvania GOP are less likely to match the views of the state as a whole and be competitive in a general election. The Growths are going to pick Pat Toomey, and he's going to lose.
There are some disadvantages to your party getting bigger in this way. If for some reason a lot of religious conservatives moved into the Democratic Party without changing their views, we might end up with more homophobic candidates. Some of them would win general elections that pro-equality candidates would also have won, and we'd end up with more homophobic Senators. There's some point at which you might want to shoo people who don't share your values out of the party, because you can win without their support. If you're a Pennsylvania Democrat and you were really optimistic about Joe Sestak or whoever winning the Democratic nomination and beating Specter or Toomey, maybe you feel this way right now.
But I'm pretty sure Pennsylvania is competitive enough that you want to be bigger rather than smaller.