Within the same manufacturer, I don't think you see a variance in motor performance for brushless motors. It was definitely a thing with brushed though.
If the motor builders were willing to work closely with the USVTA guys to ensure that the performance of the motors are close I don't see a big deal to let more in.
BUT
Here is the deal though. Novak is being rewarded with being the sole approved motor because they were willing to work with USVTA early on to produce a motor specifically for the class. No one else got on board with it.
In the end I like the one motor rule. There is nothing wrong with the Ballistic motors.