First, the locking "brake" used for parking is not known as an emergency brake no matter how convinced you are of it. It's entire purpose in a vehicle is to hold a vehicle stationary when parked. Whether or not it was effective is not the point. In rear drum brake applications, it only engages the rear shoe. Holding a vehicle still takes far less energy than it does to slow a vehicle down, Nobody with any sense would expect half of the rear drum brakes to adequately slow down a 3500 lb car especially when 60-70% of the braking is done at the front of the vehicle. It makes no sense mechanically to call the mechanism an emergency brake.