At this point I wouldn't trust him either. Giles and the rest of them are on the same path they were in the original timeline. Buffy knows that Giles never trusted Spike even after he had gotten his soul he tried to kill Spike. She can't risk anyone knowing about Spike it's too dangerous. If they find out and manage to kill Spike she'll be alone again. Only she won't even have any hope like she has now.