OK, here's a simplified explanation as far as I understand it. I make no claims of being as eloquent or educated as Drs. Jordan and Caffin. as I understand it:
An insulated, inflatable pad's primary contribution to insulation is through the prevention of heat loss through radiation. The best means for this is thick insulation to trap air and limit it's movement. Down is quite efficient for it's weight for this purpose. Notice I said limit air movement rather than prevent. There is still air moving within the down, due to the temperature gradient between the top and bottom of the pad, and this is carrying heat away from your body through convection. This convection is greatly mitigated by the down in the pad, but it is not completely eliminated.
By placing a layer of ccf between your body and the pad, these air currents can no longer come in contact with your body surface. This does not eliminate the insulative capabilities of the down in the inflatable. The thin layer of ccf will fail to retain enough body heat in cold conditions, thus the down provides for further insulative loft; the purpose for inflating the pad is to preserve this loft beneath the pressure of your body weight. Without any insulation value in the inflatable, the ccf probably won't be enough. It seems a couple of members tried using BA Clearview pads with ccfs a few years ago with poor results.
This may differ regarding the XTherm, as it uses very different principles to provide insulation.