To put you in the picture, the crews are always on standby at the stations, when a call comes through, the crews split into two groups and move to more forward positions covering the touch down and roll out ends of the runway.The reason for the split is to cover most eventualities and also not to put all our eggs in "one basket". Only problem I can see with remaining on station is the delay in getting to an incident (due to taxiing aircraft etc.)should an accident occur, plus it's a long way if the aircraft under shoots.
AFFF.