Please people, even with clouds in the sky and the moon not being full I can see in the dark just fine when making my nightly runs (jogging). Your eyes get used to it after some time and you began to see better. Cats can see just fine in the darkest of nights.
As for the caves, even dark dungeons and caverns have some light in them because light reflects from every surface it falls upon. The sewers in the prologue of TW2 for example were made just fine considering how many gaps there were where light could come through, while the mines on the other side had little to no light but still let you see the frames of what you had in front of you. It all depends on your ability to see light in different strengths.
So no matter how dark the night (unless magic is involved), Geralt should see just fine outdoors. In caves and such I agree, the further you go in the darker and there should be a point where you BARELY see anything and should use the cat potion to enhance that vision.