Me too. She auditioned for the role of Buffy but he went with the blonde. Makes sense, though; considering the Hollywood stereotype of a young blonde woman in distress; this time, she can fight back.
Well, the "Buffy" who was in the movie version was blonde. I think the "joke" is that the savior of the world (The Slayer) is a ditzy, blonde Valley Girl.
Another reason he might have gone for the "blonde" would be that Sarah Michelle Geller is a much better actress than Charisma Carpenter.
Although, I will say that Charisma Carpenter did a lot with that character and was very entertaining. I sometimes think she really got the best one-liners.