Not sure where the counter-rotation idea originated, but the very earliest Lycoming T53 helo engine had it. Later T53's did not.
Actually, counter-rotation offers a couple of advantages - One turbine nozzle (stator) can be eliminated, with weight and efficiency benefits. As pointed out earlier, the gyro loads from the rotors are self-cancelling, as are engine seizure loads for the guys who design the mounts.
One very real disadvantage is if differential (intershaft) bearings are needed - they turn at VERY high speed and are a real engineering challenge. But despite this, several modern engines are counter-rotating.
