A fridge works by heat transfer, the liquid refrigerant evaporates in the evaporator and cools it, the cold refrigerant absorbs the heat inside the fridge and changes to a gas state, it then travels to the compressor. The compressor compresses the gas in doing so raises its temperature and pressure, gas then travels to the condenser coils where it is cooled and turns back into a liquid under pressure. The heat from the fridge is transferred to the condenser where it is removed by the cool air flowing around the condenser.
If you can't remove the heat from the condenser coils no cooling will take place, the more heat you remove from the condenser the more efficient the process will be.
When a fridge is designed compromises are made due to costs, space and energy consumption and noise etc. The compressor outlet will be hot but how hot depends on how much heat is absorbed in the fridge and how much heat is removed in the evaporator, if the evaporator removes no heat then no cooling will take place.
You therefore have a balancing act, little cooling at the condenser will result in long compressor run times, lots of cooling at the condenser will give shorter compressor run times, therefore removing the most heat you can from the condenser without unduly increasing the energy consumption will result in more efficient operation. A fridge in a kitchen for instance with a large condenser in a cool environment might be able to remove most of the heat without any fans etc. If you build it into a cupboard then you will probably reduce the air flow and reduce its efficiency. Same if the environmental temperature is high. Adding cooling fans may improve this but you need to ensure the energy consumption of the fans is offset by the reduced cycle times of the compressor or you have achieved nothing.
I have found with small compressor portable fridge freezers (12V) that replacing the cooling fan with two high volume high efficiency units with one pulling and one pushing considerably reduces the run time of the compressor and reduces energy consumption, the down side is increased noise.
With my camper I found due to cabinet design the was a trapped pocket of hot dead air around the condenser, I placed a row of 5 minature industrial vans in the vent in the cabinet that run when the compressor does, these create a slight breeze, you don't really feel it but there is enough pressure created to lift a sheet of paper whereas without the fans there is no lift. These fans have considerably reduce the run time of the compressor and the fridge uses less power with the fans than without.
It really depends on what your trying to achieve, if your just want to increase the fridges cooling performance in hot conditions then a thermostatically controlled fan/s might be the go. If you want to decrease overall energy consumption ie running off batteries then you may well be better off running the fans whenever the compressor is running rather then having them thermostatically controlled. Down side is there will be increased noise levels.
