Mike Ton

IMHO, well crafted shaders address both how light reacts to the surface (diffuse and specular), as well as how the surface behaves absent of lighting (ambient).  Generally what happens when the ambient aspect is neglected, is that lighting flattens out to black.  

This can certainly be used as a great artistic effect, but more often than not it's unintentional.  As a result the spatial aspect of play spaces/environment becomes greatly diminished.  Players have to work harder to discern the play space, leading to visual fatigue.