I agree that it's harsh in practice rounds. In practice rounds you should be able to switch around and change equipment for testing and practice.
In competition, however, WGTs version of the one-ball rule is actually quite fair. It prevents sandbaggers from luring in unsuspecting victims by preventing them from presenting one quality of ball at the start of a game and then switching to a ball that better suits their game after it starts.
Even if only allowed to switch after running out or losing the last of a type, it's very easy to count the hits on a last ball and start the game with only a few hits left on it. or intentionally lose it. Then you get to use your preferred ball by the second hole after your unsuspecting opponent has committed to the game, most likely losing credits to you in the process.
Complaints about cheaters and sandbaggers are rampant. It is going to be worse if WGT starts helping them even more.