It is never a good idea to unnecessarily evacuate an aircraft outside of ramp areas, whether or not the runway or taxiway is shut down. You are correct, they will shut down and isolate a runway or taxiway when an emergency aircraft lands on it, but unless your airport only has one of both of those, it will continue to operate on the other runways. Once the paranoid passengers start the evacuation, many will just start running away to get as far away as possible. There is no one out there to direct them and many, confused, will just start heading toward the nearest buildings. Now you have people scattered across the airport trying to get to the nearest building. It is FAR more dangerous than a ramp area.