The mosfets are n channels on the ground leg of the servos.
Blocking the current flow path on the ground lead may force voltage/current to flow back thru the servo control wire back thru the arduino to the common ground between the arduino and servo power supply. If the servo starts to heat up or behave erratically, this might be the reason. I haven't tried it, but a p-MOSFET on the servo power supply might be another choice.