Just to clarify a few terms.
Rear brake is
NOT a back up system, rear break is a part of regular, standard bicycle equipment.
Long descent braking is
NOT emergency braking.
Then again, everybody is entitled to their own opinion.
Regarding front vs rear brake, below is a good, common sense explanation, based on rudimentary mechanics, of how they work and how they should be applied and understood. Hopefully some will find this helpful.
https://www.renehersecycles.com/how-...-on-a-bicycle/
"
Front vs. Rear Brake
On dry pavement, the front brake alone halts the bike over the shortest distance.
Many riders think they need both brakes to stop effectively, if only because most bikes are outfitted with 2 brakes and that implies that one should use both. Here’s the way to think about it: the momentum of your body continues to move forward as your bike is slowing down, so your weight shifts forward. That’s why your rear wheel can come off the ground when braking hard. When your weight comes forward during hard braking, your rear wheel has close to zero traction. If you apply the rear brake under these conditions, the rear wheel will lock up without contributing significantly to the braking effort.
If you can apply the rear brake without locking up the rear wheel, then your weight isn’t shifting forward – a clear sign that you aren’t braking as hard as you should!
We tried braking with both brakes and with the front brake alone, and consistently found that if we focused all our attention on the front brake, we achieved much shorter stopping distances."