Why Diffuse Light use max(N · H, 0) instead of just letting it be negative? In Cg tuts, Diffuse Section


  Surfaces that face away from the light will produce negative dot-product values, so the max(N · L, 0) in the equation ensures that these surfaces show no diffuse lighting.


My question is why not just leave it be negative? IMO, both the negative and zero will make the color be black.
If lighting with multiple lights, they add together to make the final lighting value. A negative light would darken other lights, which is incorrect.  This is also true if you add in specular reflections, emissive lighting, or other sources of lighting. The negative lighting from one source of lighting would darken some other source of non negative lighting.