I too, wondered why Kubota is concerned about BH plus loaded tires...since tires are off the ground when using BH. I've concluded (duh)
they're concerned during transport...most likely moving too fast on very rough surface, which could really stress tires. Since mine are loaded, I make it a point to transport more slowly with BH attached. I'm thankful for the extra weight...especially with the BH off. I can safely pick up more with the FEL and am a lot more comfortable in "lawnmower" mode...