An expected use-case for RectangularShadow is to be used as a drop-shadow for a Rectangle. But a Rectangle can have individual radii. So in order for a ShadowRectangle to cover this use-case 100%, being able to set individual radii on it would be needed as well.

This is especially relevant for the ongoing UnifiedStyle project, since we use RectangularShadow rather heavily for implementing drop-shadows for controls and delegates.