My big problem with the double-meld-as-gil-sink concept is that currency =/= items. The circulation of items is not nearly as tightly controlled (at least in theory) as that of currency. Monsters drop items that have value all the time but only rarely drop gil and gil drops are always small. Double-melding is certainly a VERY effective item-sink but I would say that it's a terrible gil-sink since it doesn't actually do what gil-sinks should do: remove currency from circulation.