Okay... Riley's acting out wasn't about addiction. It was about Riley's feelings of inadequacy. He said it himself: those women needed something from him (his blood) and were willing to give him a bit of a thrill in return. The important factor here was that Riley needed to be needed. But Buffy didn't need Riley. Instead of leaning on him when Joyce was sick or whenever there was any sort of trouble, she was brave and stoic. It didn't help that vamps like Angel (Forever) and Spike (Fool for Love) kept being there to pick up the pieces for her when she shut Riley out or when vamps like Angel and Dracula could put a mark on Buffy that Riley couldn't equal, either. It wasn't coincidence that Riley's whores were vamp whores instead of regular human ones. They were as close as Riley could get to Buffy's weird vamp thing. I think Riley was not only trying to feel needed, but also trying to get in touch with the side of Buffy that she didn't want to show him. This was waaaaay more emotionally complicated than addiction. Any rush Riley got out of it was secondary to the calming effect of being able to act on his negative emotions without unloading on Buffy during such an inappropriate time. Honestly, I think he was trying to protect her feelings by handling his feelings on his own rather than by laying all of his baggage on her when things were so rough.
I don't think Riley's so-called addiction made him dangerous or a bad guy. In the end, Buffy didn't, either. She obviously ran to at least attempt a reconciliation. I think what upset Buffy was the fact that she had to face her own conduct toward him -- and she didn't like what she saw. I also think she felt cheated on and cheapened, as would most women. Obviously, Riley's lust for the dark side probably made her face that part of her at least a little bit, too, and dislike it. I think Buffy felt betrayed by Riley's fall from grace at least in part because it was her own fall, too. She couldn't just be distant and pretend everything was all sunshiney and nice in the black-and-white world of their seemingly solid, stable relationship. I think it threw her that she had to work for this relationship, too, and that everything didn't just fall into place.
All of this considered, why wouldn't Xander encourage them to work things out? The problems were complex, but the solution was fairly simple. It involved Buffy crying on Riley's shoulder once in awhile and letting him into the inner-sanctum of her psyche and her core group of family/friends (as well as distancing herself from the bitey vamp boys). Given the situation, I don't think Riley's actions were unjustifiable or particularly difficult to understand. In time, I'm sure Buffy could have (and would have) forgiven him. Really, getting bitten by a few vamps isn't the stuff of apocalypses -- or anything Buffy hasn't done, herself. I don't think Xander, who can still love Willow even when she's trying to destroy the world, is one to turn his back on someone just for getting a little fang action. After all, if that were the case, he would have turned his back on Buffy long ago.
