I feel like some of the things about their lives back in the islands could still be important for the franchise going forward, although it would feel unnatural to have it being shoehorned back into the plot like this.
1. We know Sora is the typical estereotype of a clueless islander with little to no knowledge about technology and such things, although It seems like he's an exception to this rule and with Selphie and Kairi's "modern" take to their school uniforms and the way Riku acts in 3 when we first use the Gummiphone, they are probably well versed with this. So probably Sora is the kind of kid who spent his entire time playing outside instead of reading or playing games.
2. Kairi's backstory could turn out to be very important since she had access to the castle in Radiant Garden as a kid (although that probably has to do with them not wanting to model another scenary just for her grandma scene rather than making a big deal about them hanging out in the Library), something we know is unusual since Lea and Isa had to sneak inside in BBS. There are also hints that she was adopted by the mayor in DI which fits her princess role. So if they ever bother to explain how the Princesses of Heart work in KH4, that might be addressed.
4. Probably the most important detail being that Xehanort seems to originate from this world, and maybe he was even one of the original kids who built the whole base in the "Kids Island". It might also be the place where Ventus showed up when he went from UX to BBS, since that's where Xehanort leaves his body.
5. It's the place where the Paopu fruit originates from, and the stories surrounding that fruit are well know enough for Aqua to hear about them all the way in Radiant Garden. The Paopu signifies the bonds between people which directly ties into the Power of Waking. So maaaaybe it's more important than what we are initially lead to believe.
I doubt they are done with Destiny Islands yet. At this point, I wouldn't be surprised if it turned out to have a relationship with Scala who seems to be built on top of a series of small islands as well. lol