Super!Xander fics are the pretty much the equivalent of a class of Spuffy fics that I term 'Lecture Fics', in which someone (a resurrected Joyce, a Mary Sue, a time-travelling Buffy from the future, etc) shows up and lectures all the characters about how mean they are being to Spike, how ashamed of themselves they should be, and how much better for Buffy Spike is than that dreadful Angel creature or that sadistic Nazi Riley Finn.

I have read a good example of the genre once (pfeifferpack's 'Masks and Mirrors', in which the lecturing is mild, the lecturer original, and there is due credit given for the lecturees' virtues and valid reasons for their actions) but usually they make my eyes bleed.

The two, superficially very different, genres share the same underlying motivation; a belief that the writer's favourite character has been short-changed and an obsessive desire to see them get what the writer feels is their due credit and reward. I would guess that Lecture!Fic writers are almost all female and Super!Xander writers almost all male, hence the different approach (although that could me being sexist; however women do tend to solve problems with communication that men might want to solve with a pair of .50 AE calibre Desert Eagles and a commando knife).

*edited because referring to a 'twin pair' is embarrassingly bad writing and I can't bear to leave it like that.