Stenciling in Yafaray only works for diffuse color mapping. It would be quite useful if stenciling would be available for all other possible material mappings, like in Blender.
For example; this would be especially useful in scenarios where a model is covered with seamless tiling texture with global texture coordinates, as a base. To add imperfections like dirt or scratches, which also affects other material properties like bumpiness, glossiness, transparency... someone would hand-paint a stencil texture on the model. To achieve effects in desired areas (and to avoid repetition patterns), the stencil has to be mapped with UV coordinates. With such a stencil is then possible to attenuate and mask out effects in exact areas and retain the benefits of 'automatic' global coordinate assignment of tiling textures. The field of architectural visualization would benefit the most from this.
The benefit of such stencil support is also that smaller textures (faster creation process / more detailed textures / less memory) are needed to achieve a believable non-repetition result.
This would be preferred over the BLEND material also because of performance matters, as BLEND material is quite slow to render and there is need to create only one material, instead of three (or even more).