What is Advantage of Belly Pans?

[TankerIP]

For a couple of years Airstream built some units without a Belly Pan. Mine is one. The sub-floor is coated with a vinyl or fiberglass material to seal it from the bottom. I was talking to the Airstream folks who mentioned some people have had it installed (at a cost of $3500 or so). My question is why?

I guess I lose some insulating value, but this hasn't been a problem here in Texas and in the South. We have camped easily in below freezing. However, we don't expect to be camping in winter in the North. I have the advantage of being able to inspect and analyze my frame and all the components underneath. I wonder if belly pans stop water intrusions better than the vinyl/fiberglass coating on the subfloor?

Should we just leave well enough alone?

Opinions welcome.