The decision on how high the call-out should go, largely depends on how big the accident will be if it happens. If a problem escalates with a large aircraft, you will want everthing you can get. I heard that a fire and rescue bod can deal with about 4 casualties, before becoming exhausted. The airport vehicles can exhaust their contents in 2 minutes, getting additional water to them quickly could be vital. A total wipeout of a C152 can be easily handled by the airport FRS, but a major incident will need everything you can get from miles around and then some more.
The other imponderable can be the 'Capt. Speaking' no cause for alarm chat, it's not always possible to decide exactly how serious an incident could become. I would rather have dozens of incidents with outside services being sent away unused, than to have a major incident, however unlikely, being helplessly watched by too few rescuers, with empty vehicles.