I don't understand what you mean by circularity
Anyway I argue for using effective skill instead of base skill when it comes to item requirements because the effective skill "is" the skill of the user for pretty much nearly everything else that matters; crafting, dialogue, combat, stealthy stuff, for all intents and purposes nothing that I'm aware of cares about base skill other than maybe learning the Silent Isle pillar (but I cannot tell if it requires enough TC or enough Will, since I've always taken TC with high-Will builds), even stats like Perception are given the "effective is the stat that matters" treatment, spiking Intelligence lets you comprehend the Ferryman's philosophy and so on.