Oh I dunno. I thought the reason there was less Xander was because the writers were bored of him and replaced him with Andrew. And the potentials, Kennedy, Wood all popped up in season 7. I don't think TPTB would have spend time on certain characters anyway, otherwise they wouldn't be introducing fresh blood. Even if Spike wasn't around I don't think Xander would have got a great storyline. If they had stories to tell about the neglected characters I'm sure they would have included them regardless of Spike's presence. They didn't seem all that inspired though. If they should have got rid of anyone it was the potentials IMO.

The season 6 DVD had Buffy/Xander/Willow on it so that's the "core cast". I guess Giles wasn't on it because ASH wanted to leave as a full-time cast member at that time.

I do agree about Spike being included on season 3 being rather bizarre *g*