You'd be surprised where those critters can get and the kind of damage they can cause in their search for some place warm to sleep. A single rabbit making its way into a building's transformer can short the transformer, causing a surge that trips the breakers of every PDU in that building, but not before the building's UPS gets fried.
Think radiation leaks. In event of a major incident, you do not want rabbits on site burrowing around and then spreading cesium-137 and other pernicious decay products.
