I think the lesson here is that Republicans should've left Specter alone for the same reason Democrats leave Ben Nelson alone. Yes, they're crappy members of the party, but they also have constituencies to represent. Joe Lieberman had no excuse to be a crappy Democrat -- his state was quite liberal. He could be pushed. Push Specter too far to the right and you lose the seat. Or you drive him out of the party.
Update: Josh Marshall notes that Republican voter registration is way down in the past few years in Pennsylvania, which is a closed primary state. Only the real hardcores are still there. This made the 2010 GOP primary look like very hostile territory for Specter.