For conservation of energy. Because it looks better.

Let's say using some incredible physics monitoring equipment you watch 1000 photos arrive at a surface.

Let's say 300 of them (30%) bounce of the air-surface boundary, making a specular reflection. How many are left to make a diffuse reflection or sub-surface scattering? Not 1000 any more, but 700. The other 300 left already.

So - where there is more specular there must be less diffuse. QED.

There is also the hyper-color problem.

Consider a bright red plastic, RGB(255, 0, 0). Diffuse reflection from a 100% light would produce RGB(255, 0, 0). But what if you are looking at a spot that produces a specular reflection of RGB(50, 50, 50)? Then the sum is RGB(305, 50, 50) which is impossible to render. What happens is the red gets clipped so the actual rendered color is RGB(255, 50, 50).

Now what happens as you look at the surface falling away from the hot spot? The diffuse and specular are decreasing with the changing angle. But! Because of the hyper-color clipping, the red component is still 255, while the blue and green are dropping 40, 30, 20, etc. This looks strange, and produced a glowing halo around the hot spot.

I will render a demonstration. Give me a few minutes.

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