I think Xander was supposed to be the "everyman" character--the one we as the audience related to. Everyone else had superpowers, but WE don't, so we can't 100% relate to Buffy or Faith or Willow. They have aspects that we can relate to, but we can't relate to their superness. Xander, however, had the mundane, never feeling quite cool enough thing down so we as an audience had that aspect to relate to in at least one character.