That's the only way I cook because I do not have a plate setter yet. I just use a water pan with the grid extender over it. It works great and I make sure to use low temps., it's low and slow anyway. I have not did this with higher temps.
All hope was not lost. I do not have a platesetter yet and what I do is place a water pan on the grate w/water or liquid of choice and place my ribs on the grill extender. They turn out awesome and I do not have to baby sit them.