Surely there's the off chance that they do work other than Sigma-related situations. I mean, they're X and Zero; they can't just sit around for months and not have anything to do. If that's true, then maybe they are used for diplomacy? :\
If Signas was assigned to leader of the Hunters after X4, then who had general command of the Hunters? I thought I read somewhere that it was General, but that makes no sense.