The idea that recirculation and defogging are antithetical modes is not always correct. When the air outside the car is saturated with moisture - as in heavy rain for fog - the combination of A/C and recirculate (with heat as well, if the ambient temps allow) will dry the air out faster and keep it drier. The reason is that the air inside the car quickly becomes drier than the air outside; bring in a constant stream of wet air makes less sense than recirculating drier air, while continuing to remove more and more moisture from it. This was easy to prove in older cars with manually operated recirculate doors and separate A/C buttons, and I did so all the time. It's tempting to assume that the car manufacturers are idiots for not allowing this, but the more likely explanation is that they are practicing CYA when locking out recirculation in defrost/defog mode. Since there are many situations in which it is better to have the fresh air intake open, they make it mandatory.
Of course, once I discovered that the floor only setting will also allow some air to escape from the defroster vents, I was able to use 'partial recirculate' to make it more effective in damp weather by using A/C + Floor only. The only drawback is cold feet, and that can be solved by adding heat to the mix.