Test ESC using a Servo Tester
I have the same servo tester: The input does require just the supply voltage to be provided. The PCB trace for the S pin is not connected to anything else on the circuit board. 

It appears that the 3-pin connector has been used by the designer to accommodate customers whose only source of a 4.8-6 volt supply is a servo connector from either a battery pack or some other servo control equipment. 

The third pin does provide improved structural stability compared to a 2-pin connector which would be prone to shear or loose grip due to accidental pulls on the wires, or rough use. However I would hesitate to state that this was one of the thought-through reasons for the design decision.