Knowing that Faith will fight doesn't require trust, it only requires a knowledge of her personality. he only time that Buffy spent with Faith when they weren't fighting each other was at the beginning of Season 3, when Faith was a dive in blindly, fists swinging, kind of character who never considered the consequences of her actions and rarely followed instructions. Yes, we know that she had changed by Season 7, but Buffy does not, and she can only act on what she knows.
Angel trusts Faith and has little or no doubt that she will remain loyal to him and always have his back when he needs it (kind of like Buffy trusts Spike in Season 7) But Faith's loyalty to Angel doesn't automatically transfer over to Buffy, or at least Faith hasn't had the opportunity to prove to Buffy that it does, and, without that kind of assurance, Buffy isn't going to trust her with something that might prove very important in her fight. Certainly, Buffy knows that Faith will fight. What she DOESN'T know is, will Faith follow her orders in a fight, or will she go off on her own tangent again.
And, by the way, Buffy gave Faith the Scythe mainly because Faith was the nearest one to her at the time. It might just as easily been Rona, Kennedy, or even Spike, for that matter. The important thing was to not let so powerful a weapon become lost or, worse yet, used by the enemy against them.
First off, I said that Faith could've replaced from a narrative stand-point, as in plot, not as a character, but after listening to the other arguements I've changed my mind.
It could've worked. Buffy didn't like Faith, but I think she trusted her because Angel did.
And please, the whole point of the Buffy/Faith conversation in Buffy's room was meant to show an understanding, they are
Slayers, a bond that goes deeper than whatever Buffy/Spike had. Faith understands Buffy in a way Spike only wishes he could.
And saying that the only reason Buffy gave Faith the scythe in the end fight is because she was closer is nonsense. Faith ran over to Buffy to help her, Buffy passes her the scythe. It's a 'passing of the torch' moment, as in 'I trust you to hold the line, take my place'. It's a clear symbolic moment.
Also, comparing the connection between Angel and Faith to Buffy and Spike is also bull, Angel has supported Faith for
years, made her face her own darkness, never gave up on her even when everybody else had, including herself. In turn Faith would've given her life for Angel several times over (and almost did) trying to take down Angelus and capture him to bring Angel back.
I don't think anybody here would argue that their relationship is an extremely close one.
However, a lot of people argue about the closeness of the Spike/Buffy relationship in S7, that's what this thread is about, more or less.
You could see it as genuine closeness, manipulation, or a crutch for writers who clearly have an end in sight and wish to force it.
Bottom line, Buffy has known Spike, this Spike, for less than a year. Her complete trust in him comes fairly late on in the day. ('Touched')