I am not an aircraft engineer but I am pretty sure storing fuel in the wings is not good engineering. Should the fuel tanks catch fire or explode the craft would be in danger of losing a wing. There is also the balance of weight issue as an additional mechanism would be required to ensure fuel was drained from both wings equally. In addition to the fact that wings are thin and light and the leverage of the weight of the fuel would easily bend the aluminum wings. A question I would like to ask a commercial jet Pilot would be if there is a noticeable difference in the handling and fuel efficiency of the aircraft relative to expended fuel, as these craft supposedly carry tons and tons of fuel it would have to make a noticeable difference in these factors as it is expended.