I haven't registered here, haven't really thought about it actually.
When I lived in Kenya though we were always getting security warnings from the embassy... there were bombings on the coast and then the start of war in Iraq and presidential and constitutional elections. Point is, if they know how many Americans they can account for i.e., if something went very wrong and they knew where they could find the person or whether that person would be likely to seek embassy help they know who to look for and report that to DC)