Maybe I'm just a cynical old man, but, despite all of the huge and heartfelt declarations of love and loyalty at the airport, I always just saw this as Giles deserting his post. The gang had gone to great lengths all summer to create the illusion that Buffy was still alive, even re-activating the Buffybot, a less than perfect substitute for the real Slayer. Every fighter available was needed to make the illusion continue. Whether or not he was actually still employed by the Council or not, Giles should have felt it as his duty to remain on his post, considering everything that seemed to be constantly popping up there. Buffy was dead, Faith was locked away in prison, and certainly Giles knew, even if the others didn't, that there would not be another Slayer forthcoming. He was the most knowledgeable human demon fighter of the group, and, since it has been established that no one except Dawn really trusted Spike, also probably the most powerful fighter as well. The hellmouth was the center of demonic activity on the planet, he was needed there, and he chose to return home.
