I think that Angel would have simply added to the confusion and ill feelings if he had stayed, and more than a little actual conflict. Buffy already had more than enough problems to deal with in her upcoming fight without having to keep the two of them from trying to kill each other. And just imagine Angel's reaction when Buffy made him share a room with Giles while she slept in the basement with Spike. And how about when Angel burned up in the hellmouth, and Spike was left to help Buffy go and build her Slayer army?
But, seriously. that was Spike's battle, his chance to be the Champion and save the day. Spike earned his place at Buffy's side in that last battle. He earned it with his pain and his loyalty. He earned it by doing something that, as far as we know, no other vampire had ever done of his own free will, and he nearly died in the process. Having Angel swoop in there at the last moment and steal, or at least TRY to steal all of the glory, without suffering a bit of the pain or faced any of the dangers that had gotten them to that point, just wouldn't have been fair.
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