What you say has a couple merits. Trying to develop a Break Shot Stroke, while racking balls is a real test of devotion & determination. Whacking away at a rack of balls, with mixed results, doesn't allow the player to learn, because there are too many variables to consider.
Learning to make balls & control Whitey is the goal of the Break Shot, so let's do it methodically. First the player needs to develop an accurate & powerful delivery. Repetition is the vehicle that humans use to develop their motor skills. The BreakRAK allows the player to develop their stroke quickly, through that vehicle of repetititon. In doesn't take long for the players stroke to become accurate & have a punch to it.
In any equation there are variables. On the Break Shot there is a rack of balls with many variables, the Cue Ball is the only constant & the player's motions are laced with variables. But very soon, using that vehicle of repetition (the BreakRAK), the player's skill is hone & the delivery is under control... their rhythm & coordination has been developed
Once the player can hit the Cue Ball with accuracy & power, using a Rack of Balls is necessary to learn where the strike must be.
There are many facets of life where something artificial is used to train with & once the skill is there, it's off to REAL LIFE trials & tribulations...
Good Luck