My guess would be that the rash of crossovers is a healthy symptom that our fandom still lives, despite being off the air for ten years. (Okay, there have been movies, but bleh.) While there are still pure HL stories being written, there aren't very many compared to the old days. If we had to depend on our current level of pure HL fic output, we'd be pretty starved. Crossovers are like transfusions of new blood. I'm just glad for them.