Not totally true. Whether you believe XP to be worthless or not, XP on a node can be a good deterent for not posting badly thought out answers, or stupid questions. If you're XP starts going backwards, you generally end up thinking things out more. Of course that would infer that you even pay attention to nodes that you reply to, or create.
Course I'm a hipocrite talking about how XP isn't totally worthless when I'm just too damn lazy to log (for months now) because I blast my cookies on a regular basis.