I run carbon in my tank 24/7, there really is very little in the way of cons in doing this as long as you are diligent in performing water changes and regularly changing out the carbon. Carbon does absorb some of the trace minerals out of the water but they are easily replaced if you are doing regular water changes, and if you don't change the carbon regularly it can begin to leach phosphates into the water.
Carbon performs several functions, it absorbs toxins released by corals or other organisms in your tank, it also absorbs organic pollutants, and it helps keep the water crystal clear.