A watcher can indeed be useful. I've always assumed that a watcher running around is not all that unusual and since they are not stopped by buildings (if they even see the building on the astral), having one go through a building instead of around to deliver a message to someone should also be fairly common.
If we really need astral overwatch, my character can also look in the astral, so spotting spirits should be doable. I can even take them out (I have a weapon focus), unless they fly up. But a watcher can't do anything against a full spirit.