Creating a stencil from a layer of paint will always result in a solid shape, unless a tool that can create a gradation effect (like the Airbrush) has been used to add a soft effect or if the canvas grain has separated the color in some manner. This is useful for when one wants to mask an area they've painted. I use this a lot when I do comic book style images, where I first fill in an area with color, create a stencil from it, then create another layer where I add in shadows and highlights without having to worry about the colors going outside of the lines (due to the stencil mask).
When one imports an image specifically as a stencil, ArtRage makes the lighter areas of the image semi-transparent (which doesn't happen when you create a stencil of layer). The color white will be completely empty, leaving anything that's black completely masked off. This is good for making texture or gradation stencils.
Nothing is easy to the unwilling.