My understanding of the prevalence of Xander slash fics is that he's the character male audience members are most likely to identify with, so for guys who are into, say, Spike, then shipping them is a way to play out that fantasy.

Makes sense. And for women who wear slash goggles, there's definitely text that could be interpreted as subtext, and you have the whole "former-enemies-becoming-something-more" trope that is so attractive.