The old approach, because of it's use of 2 multiplied layers with 50% opacity, created odd effects when the hard shadow was overlaid on the soft shade - for example, giving darker shades when a shadow was cast on a moderately shadowed face than an unshadowed (but supposedly darker) face adjacent.
|Old Shadow Layer|
|New Shadow Layer|
To show the effect of this change, I've done full composite tests
|Old Method Composite|
|New Method Composite|
As a side effect of this change, the number of render-layers involved in the composite is reduced by 1/5th, which should cut down on render time