My brief experience with them suggests they only function when they are set to Patrol, which generally means they need to be led by someone (with magical leadership). When patrolling, they are just like any other patrolling unit, only worth many times an ordinary unit for that purpose.
I too sort of expected them to work without being led. Perhaps they should be made into zero-leadership commanders so they can patrol without having to have a "lighthouse attendant".
PvK