People discredit the Spuffy relationship. Whether you're a Bangel or Briley fan you need to understand that if it weren't for the relationship with Spike Buffy wouldn't have progressed even further as a character in S6-S7. It made her question whether or not that someone good or evil has a choice and Spike showed that with love and even caring for Dawn. The whole reason Buffy doesn't oogle Spike like she did Angel with love gestures because she didn't know till S7 that Spike truly loved her not the idea of her and that his heart was really in the battle.
Spike has, and always has had something Buffy needs (and it's not in his pants). His personality invites her toward rising to her calling in a realistic way. Not purely as hero and sacricficial martyr, not purely as warrior, but as a multi faceted human. His honesty and impatience with PC sentimentality is refreshing. He doesn't pretend to be what he is not, nor does he need or expect Buffy to be what she is not. He understands her powers, but also her humanity, and he's not imtimidated by either, instead, both aspects attract him, in both dark and light ways.
That is why he is so critical to Buffy's development. He might not have a reflection himself, but he can serve as one for her, when the others aren't up to the task, and he doesnt need or want her to be anything other than what she is.
He doesn't go whining to her to save him, fix him, etc. He knows he can survive without her, he's done it for over a century, but he also knows he wants her in his um, er...existence? lol.
Everyone else is always reminding Buffy of her responsibility to them and the whole world, always putting it on her shoulders, Spike sees her as an individual, not a role. Sure, she's the slayer, but he knows how fallible that is, having taken a few down.
In spite of his self indulgence behavior, he is more mature in many areas than the other characters, he's been around longer. He's seen more, had to put up with more. Has a sense of fearlessness in him, and the brutal honesty that centuries of existence and traveling through cultures and seeing that much of how people and demons operate will give you. He has a perspective the others lack. He is not afraid of his own darkness or that of others, he knows that it serves him and them as critically as the light side of their personalities does.
He's a survivor.
I like Spike, obviously. I'm not blind to his many faults and weaknesses, but I appreciate many aspects of his character and the vital role he serves, for Buffy, and the others when they are willing to take advantage of it.
Comparing Spuffy to Bangel or Briley is pointless, the relationship stems from an entirely different sort of need and attraction, and I'm not talking physical (though it must be a relief to have a man she doesn't have to worry about injuring in bed). It's not about romance, or finding a person to settle down and start a family with, or about being protected, or about being someone's hero, or even the world's hero.
Spike and she are evenly matched, in physical strength, in being survivors, in not having illusions about one another, in not expecting to have the perfect relationship...all that other stuff that gets in the way of people deeply connecting isn't there. It's just them, with all their strengths and vulnerabilities making contact. It's not pretty, but it's honest. And Buffy really needs a dose of honest, no strings attached, hard hitting reality and he's there to provide it. She turns from it, because she still wants the pretty that he can never provide, and the normal life, and the acceptance of her friends, etc. But when she needs honesty, He provides it.
