It would be nice feature to add to yafaray.
It is useful to simulate fall-off effects, ramp shaders like in a cloth material.
It is more advanced than simple fresnel because we can control colors acording with the normal angle with a texture.

From blender manual:

Nor - Normal
Uses the direction of the surface's normal vector as coordinates. This is very useful when creating certain special effects that depend on viewing angle

Here is a simple test:
A simple sphere gradient texture and map to normal we can simulate a fall-off effect.