I was going to add it in my earlier post, but it seemed the question had already been addressed. Like Ramair said, hubcentric rings don't really "support" any weight after a wheel is mounted and a vehicle is rolling. If I understand their usage correctly, they are only used to get a wheel centered on a hub before the lugnuts are tightened. The weight of the wheel/vehicle is then "supported" by the overall contact patch provided between the lugnuts and the lugnut seats of the wheels.
Hubcentric rings are used more for cars because it's more important to get that type of wheel/tire combo centered due to their performance requirements. A low-profile tire mounted on a sports car rim has a higher potential of being used at higher speeds, so the overall centering of the wheel/tire combo will make a bigger difference as far as high-speed vibrations are concerned.
When a person starts talking about truck usage, it's going to be almost impossible to get that type of wheel/tire combo balanced to the same degree because of the types and sizes of wheels and tires used (usually, smaller rims with larger diameter tires). High performance low-profile tires are actually balanced/shaved as part of the manufacturing process, but that's not really necessary for larger sized, and lugged, truck tires because they simply aren't designed to wear or be used in the same way. For that usage, hub-centric balancing would be overkill.
As far as using cone shaped pieces on balancing machines, they're used for a combination of reasons. As RamAir also said, one cone-shaped tool can be used for a variety of different centerbores sizes, and, using simple geometry, the cone shape also aids in centering the wheel/tire combo more precisely before balancing is attempted.
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