That is not fair, JFC does not contribute to leaking. The absent of JFC is not the reason why Nano and Zeus don't leak, instead, it is the design of the base that prevents seeping or leaking. The whole bottom part of the base serves as juice cup - this is the main reason why they are leak proof. But this kind of leak proof base design has a price to pay - the bottom air flow which is definitely more efficient than top air flow... further more, just because the air need to move from top down to the deck, space must be reserved on the side that means smaller deck diameter...
SMOK tanks don't have JFC (eg SMOK TFV8 and etc) but have similar base design like Merlin, Govad and so on, they all leak or seep sooner or later. For this type of tank, the present of JFC at least serves 1 obvious function: it helps in preventing or minimizing the chance of leaking when refill.