OK, then we'll agree to differ. I'll just say that no driver I know who qualifies in the top 30 at a National would ever run a 12th diff with any slip in it.
If you need to control wheelspin, use the throttle. A slipping diff will never get you off the line in a final, and at 0-30 in 1 second, any slip loses you places at a huge rate!!
We don't use gear diffs because they are not smooth enough for a 12th car. When Cecil Schumacher first had the idea to make a diff for a 12th car, he made a geared diff. When he took it to Wes Raynor at Mardave for his opinion, Wes said it was a great idea, but could Cecil sell it for a fiver? Realising he couldn't, and that a geared diff was not smooth enough for a 12th car (too much backlash between the gears as you go on and off power), he set about finding an alternative.
The answer he found was the mechanism we all use today that he originally saw in a machine for measuring gear forms and pitch. Since the measuring machine had to rotate, but any backlash would mean large inaccuracies in the measurement, they used this system of two ball sets between thrust races - effectively a geared diff with an infinite number of teeth, but no need for any clearance (backlash) between the 'gears'. Cecil adapted it to a model, and the rest, as they say, is history.
The original test diffs were made by turning the gear off a Mardave wheel, drilling holes in it, and making parts to hold the thrust washers. Another Mardave wheel was then turned up to mate with the thrust washer holders and viola - the ball diff was born.
The original ones sold for about £6 if I recall correctly...