If you compare the weight of the luggage system/capacity, then a rack-top bag+rack compares poorly against a Carradice saddlebag. Saddlebags also have very good aerodynamics and carry the load close to the centre of gravity. You can also access the side-pockets when riding.
With a Brooks B17 you dont need any rack system. For saddles with no bag loops, Carradice make a quick-release seatpost mounting system.