Well, it's going to save some rendering time, but how much depends on a variety of things. With all the extra time you might spend setting up the scene for such a render would probably make up the difference. If you render in color, and desaturate in post, you'd still have the color version. The desaturated version could be saved out of your video editor as a different animation entirely. Unless you wanted something (I can't imagine what) very very specific out of doing it directly from the renderer, I just don't see it being worth the extra hassle.

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Tools :  3dsmax 2015, Daz Studio 4.6, PoserPro 2012, Blender v2.74

System: Pentium QuadCore i7, under Win 8, GeForce GTX 780 / 2GB GPU.