As a vampire slayer she would already have had to spirit of a demon in her, so would that have made any difference to her?. I see it the same as having Window XP as the operating software on your computer, letting the Windows XP disc run again doesn't put a second copy of Windows on the computer, and if there's nothing wrong with what you have already it would only be a check up.
Maybe I got the over riding theme of the series wrong, but I always saw it as 'girl power'.
Letting three men have her chained to a rock and then letting them do what they wanted to her would have been against the reasoning for the series.
I agree with the girl power thing.
Maybe if the men had offered it to her with an explanation of exactly what the power would do and let Buffy make the decision, she would have taken it and used it to her advantage against the First.
But these men weren't offering her a gift or giving her a choice. They weren't even giving her much in the way of information, let alone informed consent. They had her chained up and were summoning an evil and unknown force to thrust upon her whether she willed it or no. They were using her for their own purposes. They were polluting her mind, body, and soul. They were doing so against her will. *That* is what made the power evil and *that* is why she refused it. That's not a very subtle metaphor, there.
Ultimately, the message here is that free will must never be compromised, not even for the good of the world.
Now, I, personally, probably would have taken the power. I wouldn't have been like Buffy. I'm a rather cold and unfeeling individual. My inner darkness doesn't bother me much and I tend to think the end justifies the means, so I'm not sure I wouldn't have seen the weirdo shadow men thing as a rather unpleasant means to obtaining an apocalypse free end. But that's not Buffy; Buffy draws her strength from her interpersonal connections and from her emotional ties. I agree that, in the long run, a less human Buffy would have been a less effective Buffy. I'm glad
Buffy didn't accept the slayer power boost; it would have drastically changed the tone and the end of the series. I'm also glad Buffy isn't like me. Buffy is the champion of the humans. It's probably best that the human champion retain her humanity.