why separate tanks for grey and black water?

I agree with all the posters here. Now on occasion I have equalized my tanks when I need more grey water room by opening the grey valve and leaving the black tank valve closed. This brings them to the same level. So at them time its like I have only one combined tank. But that is when I am dry camping. My TC has 46 gallon fresh tank and I know I can run it dry with the capacity of both tanks being used. But usually I keep them separate.