Even though the sand pad drains quickly it is also sand that gets messy and gummy.
I put TWO ground cloths under my tent at Ft. Wilderness. One is the thin painters plastic stuff that I just ditch when I break camp. The other is my standard ground cloth about an inch smaller than the footprint of my tent that is 6mil plastic.
I also put one inside my tent for extra measure.
Even at Ft. Wilderness I have seen small amounts of water underneath that plastic inside my tent.
Carol