I really don't see it. I see a friendship, similar to Angel/Faith. In the first place, Faith is in no place to be in a relationship with ANYone, and yes, I mean even more so than the other twisted characters on the show who are, relatively speaking. (I haven't been reading Angel and Faith, so I can't speak to her current state, though).
Spike and Faith have a lot in common, particularly in season 7. Faith has just returned from prison, where she went, voluntarily. She is trying to redeem herself to Buffy in particular, and prove to herself that she is something other than who she was before. Spike has just returned from Africa where he went to get his soul, voluntarily. He is trying to redeem himself to Buffy in particular, intent on proving that he is not evil; that he is now "the kind of man who would never." Both did horrible things specifically to the people they cared about most, right before making that first step toward reforming, (prison, Africa).
Nobody but Buffy has much faith, (haha), in either one of them. Neither one has much to live for but both want to be useful before they die.
Spike, historically, is not particularly attracted to those with which he has a lot in common. Certainly, with Buffy, we see that it's her goodness that captures his attention; even before his soul, in season 5 he fights to be accepted into the "light" with her--and when that fails he tries to bring her down in to the dark with him, in season 6. Though he tells her she's just like him--that she belongs in the dark--we know that that is a desperate, (and truly upsetting if not exactly par for a vampire), tactic to draw her to him. A last-ditch attempt. But what he's truly drawn to is that unattainable good that ultimately keeps her from being stuck in the darkness with him; for some reason he's drawn to it as early as season 5.
Faith doesn't have that. In season 7 when Spike looks at Faith, he may as well be looking into a mirror. Just take the beginning of "Dirty Girls:" Spike mistakes Faith for "evil," and Faith mistakes Spike for "evil." "Isn't she supposed to be the bad slayer?" he wonders. "Isn't he supposed to be a soulless vampire?" she wonders. And all around them are scoobies who are wondering the exact same thing. "Soul, no soul...prison, no prison...how do we know if they've changed?" Anyway, all this is not something that sparks a desire for a relationship within Spike.