I get it that such building was for detecting the skaven undercity in local region, but is such detecting ability really necessary? Takes 8 turns to build and uses 1 valuable building slot, why don't I just sent an agent to that region instead? And to deal with skaven, the only thing I need to do is to capture their settlement anyway. It's not like they could spawn stacks of army out of no where(via underground city)
I do build those extra movement range buildings in certain regions depends on either that was the traffical nexus for my army or not in the current campaign phrase, but this, Night Watch, has never reveal its value to me.