Very often I'll just drag that saturation slider in Aperture all the way to the left to create dramatic black & white photos of my sports photography shot under stadium lights. These two photos were shot under the lights of a high school stadium (meaning: not very good stadium lights). I think the black & white affect is a dramatic usage of this conversion that helps to highlight aggressive sports effort particularly at night when high camera ISO introduces grain in your images.
These photos were shot with a Nikon D700 and 300mm, f/2.8 lens. ISO: 6400

