Spike's concern for Drusilla must mean more to him than staying around in a world of walking Happy Meals®—and simple sanity—since he protects Drusilla by saving Giles at the cost of G Iles' likely revealing to Angelus how to send the world to Hell. When he leaves with her, his vampire nature is also getting the better of him, because as amusing as it is to see him abandon Buffy, to save the world he really ought to be fighting with her.
Spike's concern for Drusilla must mean more to him than staying around in a world of walking Happy Meals®—and simple sanity—since he protects Drusilla by saving Giles at the cost of G Iles' likely revealing to Angelus how to send the world to Hell. When he leaves with her, his vampire nature is also getting the better of him, because as amusing as it is to see him abandon Buffy, to save the world he really ought to be fighting with her.