As a web developper, I already had the request to include grayscale thumbnail images in a theme. For exemple, in a gallery, the current image is in full colors and other images are in black and white.
WordPress could allow theme developpers to add a new image size and specifying that the new image should be in black and white.
add_image_size( $name, $width, $height, $crop, $desaturate )
(boolean) (optional) Desaturate the image to have a grayscale thumbnail.
Drupal 7 allows to do this with the desaturate effect.