Like I said to Muke, that is an argument that could be made for any world that we may have seen before in the series. That is limiting the possibilities of what the story can offer and what role any worlds could play in KH3.
The fact that we don't know anything about Destiny Island is enough to leave it open for a ton of exposition.
We know that it is a world that rests close to the light, has a fruit that is said to tie people's destinies together and somehow people across the galaxy know of the legend, that our heroes came from that world and our main antagonist also came from it, that so many people are drawn to this world because of its light and the light of people who live there, that it is a peculiar place where so many begin their journeys and return home to, and the most important place to Sora, Riku, and Kairi. A place that Sora and Aqua have fought insane battles in that were pivotal in saving the universe from falling to ruin.
Why wouldn't there be something worth visiting this world for in KH3? It is the perfect place to learn more about Xehanort, the perfect place to visit before the inevitable final battle in the Keyblade Graveyard for the main cast to remember how it was DI that started it all for them, the perfect place to add more FF characters, and I think I could probably go on...So I'll stop there.