Sawtooth:
Inside a wall of a building "R" values of building insulation are cumulative on a 1:1 ratio; R1 + R2 + R3 + Rn = RT.
The Thermarest NeoAir and the Big Agness Insulated Air Core mattresses are both filled with down so they provide insulation and they have kept me warm sleeping in a tent on the ground and also sleeping on a cot, in single digit temps. So IMO you don't need the foam pad to keep warm.
However, I do just what you are considering. I sit on half a foam pad during the day and I put it under my air mattress at night. If you lean on your elbow it will go through the air mattress and hit the hard ground below making for sore elbow. This problem is alleviated by putting the foam pad under the air mattress.
Another reason that I use the foam pad is, like Rock Chuck said, to protect the air mattress and hopefully prevent leaks.
KC
