What kind of underbelly material are they talking about? Is it the nylon tarp material with insulation stuck to the inside, or is it the corrugated cardboard-plastic type with insulation stuck to the inside? I've had both, and perfer the corrugated cardboard plastic type as opposed to the tarp style.
Getting that cover on it will save a lot of dirt from kicking up on your frames, plumbing, wires, and everything under there. Much easier to simply spray down with a sprayer on a garden hose, then attempting to clean the frame, wires, and everything else.
Insulation value? Uhh? who knows. My trailer claims to have the "Arctic Package". All I know is, a lot of heat goes under there that I wish went up inside the living space of the camper.
I still prefer the underside to be covered.