Values make the portrait, not the colour.
In portrait painting value contrast refers to the relation of lights and darks. This contrast is more important than the colour contrast.
So in portrait painting it is really fundamental to pay attention to this mutual relation. It is the cornerstone to paint a good portrait.
Consequently, the importance of good shadows is consistent. If there is hardly any shadow on the face, like in frontal lighting, the resemblance must be made out of subtle colour changes. That is more difficult than with distinct shadows.
See also : What is Chiaroscuro?