Wait a minute...is this a plot-hole (Austerities of Wind and Main Scenario-related)?

[RomildaRayne]

Exception being Bahamut come Heavensward, apparently.

By the time period of Heavensward, the Warrior of Light has already cleared the Final Coil of Bahamut; it's part of the story, even if not a mandatory one for each and every player.

Don't be surprised if the game acts as through you've defeated the Coil storyline regardless of whether or not you actually did; World of Warcraft does the same because it's easier and more sensible not to make the effects of raids only available to the raiders able to complete it and it's very rare that completing a raid will make a difference in how people treat your character.