the thing is- it isn't a real gil sink. all it does is redistribute the gil to the people selling the materia. (and i suspect there are a lot of RMT PL/burn parties spiritbonding stuff day and night with this explicitly in mind)
if square wants to remove gil from the economy before 2.0 through relic quest (and they should absolutely try), all they have to do is make the materia purchasable from an NPC for a reasonable price. even if it's 500k each, that's still better than the 1-3mil/ea. prices some materia are trending toward.
poor design.