Originally Posted by 390beretta
...........What am I missing here?............
You are missing the rotational inertia of the engine.
If the braking is quick enough, your engine may be fighting your brakes during the time it takes for rpms to drop down to close to idle.
If the braking is less than an emergency, engine braking is effective, especially for slippery conditions, as the braking effect is pneumatic (more "cushion") rather than mechanic-hydraulic.
The other reason, as well explained in previous post, is to be ready to resume movement as needed, in the proper gear (yes, you should downshift during the emergency stop if you can) and with the engine on.
It is spelled in the MSF Basic Course handbook; check Unit 3 - page 23: