Don
Cannot agree! A safety pilot is just what the description implies ie He/She is a backup pilot/pilots to the required pilot/crew for a particular aircraft.
If the aircraft is single pilot then the safety pilot is if you like it a spare should the unthinkable happen.
The safety pilot could indeed be a passenger.
It is common practice to have a passenger in the right seat on a single pilot aircraft. If that passenger happens to also be a pilot capable of flying the aircraft or assisting the required pilot if the going gets tough he is then a safety pilot.
He is not required to legally make up a crew for the aircraft and infact may not be used at all during the course of the flight.
A larger aircraft which use a crew may carry safety pilots as passengers.
Some very wealthy owners not only carry a spare crew but their own Doctors and medical equiptment.
Pace