contractors can get on the surface of planets, like corpmates, once there they can use splash damage to destroy defenses. this makes it easier for a non contracted third party to invade.
if you are contracted to someone, they can /c your location and tell people where you are to set up ambushes.
contractors can access the refinery as well, and there is all sorts of havoc you can wreak from in there. colonies need metal ore to fire their solar cannon, so just sell it all on commodities market to render a ten shot a no shot in seconds.