We rebuilt the rear axles in my M3 - probably had right at 300,000 miles on it at the time. It was either $200 for the rebuild kits or $1200 for replacements.

You could tell there was some wear on the cages and bearings, which there should have been at that point, but nothing that looked or felt worn.

Based on that experience, I would say the only reason the cages may get modified at all is to make the reassembly, especially on the blind, or closed axle stub end, easier.

My early thought is the rule was exploited though I'm not convinced it created an advantage, but my experience is no wear would open up the cage enough for a bearing to pass through. To be fair, the Miata joint could be a different enough design.

And I can't imagine having to drag a tub onto the open trailer to get the race car home.