With a single compressor model, when determining how cold you want the refrigerator side, you are simply allowing more dry cold air in. That dry cold air needs moisture and finds it with your fresh food and dairy products! With a dual compressor system, one unit produces dry cold air for the freezer and the other is produces moist humid air for the refrigerator. This means no mingling of different air temperatures and humidity levels. With each compressor splitting the workload, a Sub Zero will last three times as long and will save you money by preserving more of your fresh foods.
Here are two pictures of Sub Zero appliances in both in the stainless look and with cabinet panels. As a designer, I prefer to put cabinet panels on the refrigerator to minimize some of the stainless domination plus it’s easier to keep the surfaces clean.
Images: Sub Zero Built-in Refrigeration Design Guide