>If you cannot see detail below zone 0 nor above >zone 10, what does a greater range than 10 stops get you?
In the same way that published mileage ratings for a car are valid only under optimal conditions, the dynamic range of a camera is typically valid only at low ISO settings. The DR of the D600 will be far less at ISO 6400 than at ISO 100, as you see in the chart published by the OP. If I start with a better DR at my base ISO I should have a better DR at higher ISO's. This can be very useful.
For example, let say I'm shooting football outdoors on a bright sunny day. I'm shooting with a 300 mm f/4 lens using a 1.7 TC (f/6.7 effectively). I want the shutter speed to hit at least 1/1500 sec. to stop motion and so I set Auto ISO to move ISO up as required. This combination can easily drive ISO 400 to ISO 800, and my dynamic range falls. However, I have a football player with white uniform stripes and a face in shadow. I either overexpose and blow out the uniform stripes to ensure the face looks nice, or I do extra work in my image editor to lighten the face. That's a pain when you're editing a few hundred images! The extra D600 dynamic range would be very helpful in this situation.