I'm surprised it is thought to be effective. Loading time must be considerable. The 747 is not a beast to be landed quickly, or departed quickly with the potential for massive trim problems. Frequency over target would not be very high. Now that Russian Beriev flying boat, loading up by skimming over a lake or in a high speed run in the water, then lifting off would be back over target far more rapidly (if there was a lake!) with a good load.