*Quote - "Now common advice says that the diffuse component should be at most around .8. So what does that mean? It says that whatever spot you're looking at will be .8 times the luminance of the light.

But think about that. If you render this object from some viewpoint, you see roughly 80% of the light luminance coming from the object. 80% of the light is bouncing towards this camera position. Now move the camera and render again. Here, too, 80% of the light is bouncing towards this new camera position. How can this be? If 80% went to the old position and 80% is also going to the new position, then 160% of the light is going to these two positions collectively. That's impossible, right?

In fact, if you mounted a thousand cameras in a hemisphere around this object, you'd find that all of them are getting 80% of the light. What does that actually mean?"

*Light will be hitting the object from all directions. Only some rays (or some photons within a ray) will be reflected towards any one camera. Each camera is receiving 80% of the light that would have been reflected in its direction had the surface been 100% reflective. Your second camera is receiving 80% of the potential light too, but it will come from different light rays than those that are reaching the first camera.

You have the old 'if a tree falls in a forest when there's no-one there to hear it, does it make a sound' conundrum going on here. If you have no cameras at all, is there now 0% of the light going anywhere? And if you had 10 people in the forest, would that make the sound 10x as loud because the aggregate of the sound waves reaching everyone's ears would be 10x that reaching the ears of just one person?