At the same time, I think that if something had happened between them -- say in season 7 -- it could have been great. At least in my experience, there's a difference between dating your friend in high school and dating your friend when you're older. The guy with whom the perky, petite blonde is "just friends" in high school is the guy she doesn't respect. He lacks something superficial (like "cuteness" or social standing) that she thinks is essential if a guy is going to be worthy of dating her. He's "beneath her." That changes when she gets older. The guy she's friends with when she's older is the guy whom she respects and admires too much to take the risk of ruining it. That, or the timing isn't right. The good ones are usually taken.
I was the perky, petite blonde (I actually look a little like SMG, only minus the stylists) who took the risk both times. In high school, it was a disaster. The friendship did not survive. Later, in my early 20s, a long-time friend became available at around the same time that a relationship of mine ended. We both tried to deny it for a long time because we were afraid of ruining the friendship (and there were some other factors at play, too). Ultimately, we took the risk, and now we're married. Best decision we ever made. A romantic relationship that is built on a longstanding friendship is truly ideal. Think about it: no awkward getting-to-know-you phase, no awkward sexual history conversation, and no worries about "meeting the parents" cuz they've already met years ago. You skip all that stuff and jump right into a relationship that features the comfort and trust that only come with time.
Based solely on my experience, then, I think a Buffy/Xander relationship could be great if Buffy would drop the Xander block from her brain and be willing to take the risk. But that's a big risk. After all, Xander's the only guy who's never left her. If she ruins that... umm, bad. So I don't know if Buffy would ever take that risk. Something big would have to prompt her.
