However, I haven't been shown what Buffy sees in Spike.
I have. I'll have to think about it though to see if I can put it properly into words. (I also, btw, don't think you can look at any Buffy season in isolation-what Buffy sees in Spike includes what she came to see him in season 5, and the Spike she confided in in early season 6. The AR isn't the only thing in their history together.)
In the meantime, I liked that last scene between Buffy and Spike for two reasons. One, it wasn't about what was said; it was really about what Spike couldn't say, and Buffy wouldn't say. Spike, all his protestations to the contrary, is still in love with Buffy, and was looking for permission from Buffy to tell her so, and Buffy, aware of that, refused (for now anyway) to give it. And second, because Buffy's refusal to let Spike admit his true feelings is exactly what opens the door to the FE in the final scene when Spike is spying on Angel and Buffy. Spike claims he has no expectations, but that scene made clear he still has hopes. And jealousy, which the FE hopes to use to turn Spike yet, depends on hope.