At the point where it would have become necessary to sacrifice Dawn in order to save the world, the battle would have fought and her death would've been the only thing that would close the opening between the dimensions, thereby saving not only our world, but all the others. At the point where it became necessary to sacrifice Spike to close the hellmouth, she did it. Rather than sacrifice Dawn, Buffy jumped in her place. Rather than tear the amulet off of Spike's neck and dragging him bodily out of the hellmouth, she left him there to die. She HAD learned, and when the moment came, she DID sacrifice Spike to save the world.
Before that time, Spike was, as she told Giles, "the most powerful fighter we have." Spike is the ONLY single person she had ever fought who was so close to being her equal that the outcome was always in doubt. (Well, there WAS Glory, but she got beaten in the end. Okay, Giles strangled Ben, but the good guys won in the end and Glory died.). Until the moment when the final decision became necessary, keeping Spike around was a calculated risk, certainly, but one that, as the leader, she had every right to take, even if it had meant the death of one or more Potentials, or one or more of the Scoobies. As a fighter, even when Faith showed up, Spike was still her strongest warrior. When the time came, when the choice had to be made, she made it, hard as it was. She left Spike to die in order to achieve victory.
Buffy was right, Giles was wrong. It was Giles. not Buffy, who refused to see the big picture. If Giles and Robin had been successful, the war, and the world itself, would have been lost. I sincerely hope she rubbed their noses in that fact for the entire time gap between the end of Season 7, and the beginning of Season 8, because they most certainly deserved at least that much. Buffy had no time for vendettas, and that is exactly what both Giles and Wood were trying to carry out, Robin for the loss of his mother, and Giles simply because he "wanted something better," for Buffy, as though that were his decision to make.