What are the benefits of loose, rolling axles?
Sorry for the delayed reply to this topic but SpyDude is onto something there. Since we're dealing with 'unfixed' wheels, a limited amount of side-to-side movement(mechanical term: 'lash') of each spinning wheel appears to be vital to it's overall performance. On most cars, the wheel 'lash' becomes adjustable when the axles are loose. However, I believe there exists a happy medium where the axles are loose enough to adjust with just enough resistance to hold their position when the car taps the siderails.
On that note...the older Chinese-made BMW M1s are known for having super tight axles right out of the package. My examples require the use of needlenose pliers just to budge them.
I am of the opinion that Mattel puts rotational axles in their cars mostly to put more money in their own pockets. When Hot Wheels first came out their axles were L-shaped on cars made in the US and U-shaped on those from Hong Kong. In either case they were rigidly attached to the chassis,they functioned as miniature torsion bars, and if they became bent they were relatively easy to straighten. If the rotational axles on today's Hot wheels become bent the chassis anchor rotates to the axle's lowest point, and the wheels show negative camber. The tires then rub against the car slowing it down. If the axle isn't first held from rotating (it can be done, but it's a challenge) any attempt to straighten this axle causes it to rotate in the direction of the bend worsening the problem.
Thing is, on a road course, loose axles are going to mean the wheels and weight will be shifting back and forth. It might not seem like much, but that 1-2mm of shift could come into play between keeping a straight line through curves and wiping out.