War is not about killing every man on the opposing side; it is about killing enough men on the opposing side that none of the others have stomach for more fight. Part of winning that contest of wills called morale is retaining sufficient quantity of ammunition. A unit of archers without arrows is nothing more than prey.
So while it is useful to have every archer of a unit put a dozen or so arrows into the air in rapid succession, it is expected that the shock of the resultant casualties will cause a morale break in the approaching force. Once that happens, firing more arrows is worse than useless, as those arrows are more effectively reserved for staring down the enemy, or firing at an enemy unit that has not yet been shocked into a morale failure.
From this I conclude that while being able to fire that many arrows is a useful skill, it a skill that should only be exercised in the most dire circumstances, against an enemy of extremely high morale and armour. In less dire circumstances it would simply be a waste of ammunition.
I believe that an important assist in forcing a morale break on the opponent is making going forward more dangerous than going backward. To this end an archer nit would want to fire sufficient arrows to shock the oncoming opponent, and then dare them to repeat. Shock them again every time they recover and continue, until they get the message.
Any unit with the reputation of firing after a retreating enemy has actually suffered its own morale break - by making regress as equally dangerous for the enemy as progress.