My suspicion (and I'm not set on this) is that Buffy can't judge people close to her. I think she has fairly good instincts when it comes to people (and non-people) who don't actually affect her worldview. With Spike, it isn't just that she has to deal with the idea that he might be good--she has to deal with her feelings for him as well, and her feelings are tied up in believing that he's bad (and so she's bad). Similarly, she has a hard time seeing Willow as capable of dark in season six, or even that Xander was capable of suaveness in season five.
With Spike, I think it's interesting that the Scoobies are probably more accepting of him in season four, when he doesn't represent an emotional threat to anyone--he's a distant enough figure that they can consider the possibility that he might be able to do good, c.f. Giles suggesting to him in "The I in Team" that maybe there's something significant to him getting a chip. It's only when Spike starts crushing on Buffy that he becomes a threat again, emotionally to Buffy and consequently to her friend. Gotta see when I rewatch season 4-6 how it changes. (And I think a big part of that is none of the Scoobies want to relive the time of Angelus!)
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With Spike, I think it's interesting that the Scoobies are probably more accepting of him in season four, when he doesn't represent an emotional threat to anyone--he's a distant enough figure that they can consider the possibility that he might be able to do good, c.f. Giles suggesting to him in "The I in Team" that maybe there's something significant to him getting a chip. It's only when Spike starts crushing on Buffy that he becomes a threat again, emotionally to Buffy and consequently to her friend. Gotta see when I rewatch season 4-6 how it changes. (And I think a big part of that is none of the Scoobies want to relive the time of Angelus!)