Not sure about the A330/A340, but the A320 used to have the same design and now the flap no longer exists. Decreasing the cavity area does reduce drag, but the cost of maintenance of the flap outweighs efficiency gains.
Quick refresh on what the exhausted air is. The aircraft uses air taken from the engines or APU to pressurise and regulate temperature in the cabin. This air of course needs to be cooled as the majority of people can't survive a blast of air at 800c. The cooling of the bleed air is achieved by a mix of heat exchangers, compressors and turbines.
The heat exchangers in this case are air cooled (Like a porsche). On the ground air is draw-in and over the heat exchangers by a turbo fan, thus the FULLY opened flap. In the air the aircrafts forward airspeed achieves a great flow rate over the heat exchangers and the air is cooler, meaning the flap can be almost closed and nearly flush with the aircraft.
Hope that explains it!