Educate me as to why having an extra 1/2 hour of fuel on board would make you overweight. I understand you would be flying unnecessary fuel to DCA but would you be overweight?
You expect to fly for XXX minutes and burn XXX pounds of fuel. If the flight gets shorter and you burn less fuel the plane is much heavier landing. This has to within the limits of the plane, requires a longer runway, potentially more wear on the brakes. If rain or inclement weather happens, then the runway distance is even longer and DCA isn't the place you want to chance runway distance.