I am always reminding my art students to use as many values as they can in their art work.  I stress the importance of a value  (lights and darks)  study chart to make sure that they are aware of the many values in reality.  If you have a great range of values - from light to dark - your work will have more depth, thus look more "real".  By applying a full range of values to your drawings or paintings your art will take on a more 3D ( 3 dimensional)  appearance.  I always reiterate that if you limit your value range your drawings or paintings will look flat, and you don't want your subject matter to look flat, EXCEPT for shadows!   Think about it, shadows aren't real, they have no depth, hence they are not 3D.  So, it makes sense to render shadows with the same value -  not a range of values.  You DO want your shadows to look FLAT because they are!  I often use my painting below to illustrate this concept.  Notice the shadows in the painting, those of the hanging plants, barrels,  and the light fixtures.  They are flat against the house, thus they are painted in the same value.  They only time they change value is when the colors of the house change.  If these shadows were rendered with a range of values they would take on a life of their own and appear 3D.   That would lose the effect of the shadows.