Softening shadows and lightening shadows are really 2 separate concerns and they deserve separate controls. The Soft Shadow Diameter setting affects the softness of the edge (it adjusts the diameter of the light source being simulated a - a total point source would create totally hard shadows). At the moment there are no explicit, generalized shadow lightening functions, but you can use the Enviro Light Effect On Surfaces to achieve a somewhat similar effect.
- Oshyan