To borrow a phrase from a series that shall not be named, G'raha is "still in his promotional period." It's easy to forget that Alphinaud and Alisaie had theirs, too, in the leadup to Heavensward and Stormblood respectively. Krile also got hers in the form of the Triad story, but then she basically took over as a less intrusive Minfilia and continues to dwell primarily in side stories.

We don't really know where G'raha will land after 5.55 (or if he'll even still be breathing), but there's a distinct possibility that he's still around the way he is because we need somebody else who remembers Emet-Selch. Their time together in Aumarot could be one big Schrodinger's Gun as far as the writers are concerned.

(That link is to TV Tropes. Do not click it unless you are prepared to get lost for a few hours.)