First off, the wheel studs do not support the weight of the Jeep. Somewhat fancy physics explains this on a Miata, similar concept applies to Jeep just with 5 lugs instead of 4. Once you torque the lug nuts down, the static friction between the wheel and the hub support all of the vertical weight of the Jeep. On the 4 lug example below the static friction between the wheel and the tire is capable of supporting 6000lbs per wheel. The YJ has an extra lug nut so the load it is capable of supporting becomes 7500lbs per wheel. The numbers will be slightly different due to different thread pitch and lug sizes but not enough to make up for the fact that the load it is capable of supporting is much much larger than the weight of the Jeep. The lugs do support the lateral forces in both situations though, so any side hill or turning the uphill/outside lugs will have a tensile load on them.
Guys name is Lance Schall
pg 1 http://www.idsfa.net/miata/lugnuts/page1.jpg
pg 2 http://www.idsfa.net/miata/lugnuts/page2.jpg
So what it comes down to between hub and lug centric is when you mount the wheels. Hub centric you put it on the vehicle and the hub centers the wheel. Lug centric you put it on the vehicle and the wheel just sits on the wheel studs and is not centered, the lug nuts will center the wheel though that is why they are conical at the bottom, but it is still possible to mount it just a little off center if you torque one lug before the others. So the only difference is when you mount them.
Also most hub centric rings are made of plastic, so how much weight do you think they would really support before failing?
So my guess as to why people prefer hub centric wheel spacers is for the ease of centering the wheel. With a wheel spacer you have to center the spacer on the hub, and the wheel on the spacer. If the spacer is hub centric it is much easier to ensure that it is centered correctly. With lug centric a little bit off on the spacer to hub and a little bit off on the wheel to spacer could cause a large vibration.