Prevents diff meltdowns
The single contact point from a rectangular hole ensures that the ball is rubbing up against the gear at only 1 speed(1 contact point = 1 rotational speed).
If the hole is round, the ball may contact at more then 1 point(depending on how close in dimensions the hole and ball are), let's assume the hole IS much larger then the ball. There will be a larger contact point, as the different parts of the ball are moving at different speeds (rotation is same, but actual speed is different). The ball will be causing the gear to overheat because of the differential in speeds. Just imagine stepping on the gas pedal in your car and applying brakes at the same time. Something is gonna give.
The single point contact actually makes it more durable.
Properly built, of course.
-Alexander
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