I don't think it would work. Not that they could be a healthy couple anyway, but a less perfect couple, sure.
I keep seeing Spike take advantage of Xander, like he did with Buffy, making Xander feel dirty and hate himself, when Spike got tired of the "good guy" act. True, Buffy was in a dark place, being suicidal and depressed, but at least she could punch back.
If Xander loved Spike back, there might be no need for Spike to try to "mold" Xander, the way he tried Buffy. But I keep imagining the bathroom scene. People can be raped without there being physical pain involved, and like I said, Xander can't punch back the same way.
I'm not saying that these worse case senarios would have happened on the show, but I don't think any writer could have realisticly have written Spike & Xander in a relationship without showing Spike's darker side. Which makes me hope that Xander, who hopefully respects himself and Spike's past victims enough, would say "No, this isn't working, I need to leave you".
I could be wrong, but I both don't want any "good" character to be with a soulless killer, or believe they should be able to. So in the long run, it couldn't have worked, unless Spike got a soul, or went through the same thing Anya did in Selfless, which I don't think he could. He was just a different kind of evil.
I have been wrong in the past though, so maybe they could have worked as a healthy couple. I just seriously doubt it.
That being said, I love reading fanfics with these two. Both dark ones and fluffy ones, realistic or unrealistic.