Wanderer, I see what you are saying, in my family we've always used the plastic under the tent to stop little holes getting in the actual tent floor, so that when huge storms have all kinds of water running under a tent, at least we (hopefully) keep the integrity of the tent floor pretty good. I can see how the plastic inside would keep stuff dry, but I'm in the camp of trying to keep the tent floor unholey, and has worked pretty well for me so far.

Oddly enough, the worst surface I've put a tent on was last summer, on a supported trip we did. One night the small village we were set up in had us camping in a farmers field, and the stubble of whatever crop it was, was way worse than stoney or "sticky" sites I've camped on. It seemed no matter how you tried to lay the stuff down, there were hard pokey stubble bits trying to put holes in both the plastic under sheet and the tent floor. Ended up having to kind of coax them all down sideways as much as possible before laying down the campmats and panniers inside.
I've always just used a section of bed mattress bag plastic or whatever, tough and cheap.

cheers