I agree with Cordelia and Epo...
My argument stems from watching the end of season seven.
First of all, Buffy is the only one who does (and possilbe can) pull the scythe from the rock. Now, I'm not sure if Faith could've also, because we didn't see her try, but from that perspective, it seems that Buffy is the one who is entitled to it.
Second, because that's not always proof enough, I think back to Chosen. In the huge fight in the the Hellmouth, it appears that the fight is not going so well as soon as Buffy is stabbed and falls to the ground, apperantly out of the fight. As soon as that happens, you see all the newly made slayers, and Faith begin to succumb to the ubervamps, getting taken down. Here, I make an obvious connection that when Buffy goes down, so does everyone else. Why? I assume that it can be explained that because Buffy still holds the slayer line, all other activated slayers... who are not suppposed to exist at this time, follow her physical action. When she is defeated, they are defeated. The second that Buffy stands up and wipes out three vamps with the scythe, you miraculously see the other slayers begin their comeback.
That's my story, and I'm sticking to it as well!