I think Joss uses each of Buffy's boyfriends as a cautionary tale to the young females in his audience.
Well he's on record saying that Parker was her most important partner... *g*

I usually just put my fngers in my ears and go "la la la" when Joss tries to tell me about Spike and Buffy.
Pfft! He's totally a Spuffy shipper - listen to his Chosen commentary. During the B/A scenes he waffles about lighting and how great the actors are. During the B/S scenes he All. About. The. Characters! (He's just also a bastard, hence the no-Spuffy ending of NFA.)