If someone asked me how do I decide to process a photograph as black & white, rather than leaving it as the original color image, I would say the main factor is the amount of color in the original. If the original image itself lacked a wide color pallette, or virbrancy, then I would start to consider converting it to black & white.
In the side-by-side comparision shot above, the origianl color images of the tires was nearly devoid of color, save for a little green coming through the fence. Also, the main subject, the tires, were already themselves black, thus the photograph presented itself as a good candidate for black & white processing.
So if you have a photograph without much color pop in it, think about converting it to black & white as a way of best presenting the image to the viewer.