Wensleydale has it spot on. And even when E-3D is used on a national basis or on coalition (vice NATO) ops, the availability of boom tankers is far greater than probe and drogue. It's also a much more efficient way of getting the fuel on board (up to 6000 lbs/min), so less time off task. Incidentally, when the UK decides to use E-3D for non-NATO ops, the aircraft have to be down-declared from the NATO ORBAT so that they are formally unavailable for tasking by NAEW&C FC.