You might want to be able to focus at full aperture then shoot stopped down without having to change the aperture value on the aperture ring. Stopped down metering also tends to be inaccurate at very low light levels, and with high magnification low light levels are the norm, so you might get a more accurate meter reading by metering wide open and then stopping down. Lacking a double cable release, of course, you could just meter wide open and stop down manually. You can also just use two cable releases, stopping down the lens first and then firing the camera.

Double cable releases are often adjustable, so you may be able to get them pretty close to in sync.