Unless Charisma or Joss ever give a statement, I doubt we'll know what really happened behind the scenes. It could have just been a rare mistake for Joss, the whole Cordy ascencion thing, but that might be oversimplifying things.

I loved Darla and Dru, and I loved when they got the vamp foursome together. But that being said, I never liked Spike. I did prefer him as a villain, rather than a hero, but I never got, or was even comfortable with, his relationship with Buffy. I wasn't too pleased when they announced they were bringing Spike onto Angel in season 5, even though that was probably one of th factors that led to the show's renewal.