I'd also like to add you say only Ceres and Terra have a Fundemental relationship with the empire to drive the plot forward, but the same is true of FF VII and cloud and Aeirth. Red XIII and Cid could be dropped from the story entirely and the overall plot wouldn't change in the slightest.

I'd go into details on other FF's, but this is something that holds true for virtually every RPG ever made, not just the final Fantasy games. Very rarely are even half the casts necessary to drive the plot forward.

Hell, you have a game like Chrono Trigger who has what, one character with a direct relation to Lavos? And he's optional. As good as individual stories are among the cast, really none of their individual stories matter all that much in terms of the greater goal of the story. They are just a bunch of people who see the end of the world, and decide they should do what it takes to stop it. Not really anything more then that. And the story doesn't suffer in the slightest for it.

I guess in the end I fail to see how this is even a problem. So long as the plot can justify all the characters being there reasonably, I really don't take issue with it.