I mean in retrospect it was overkill and really ended up making things more complicated than it needed to be, but I understand why Nomura wanted to make handheld title. Without the games, the KH franchise would've been dead until 2013. I feel like BBS should've stayed as a PS2 title (or PS3 since it came out in 2010), but Days and Coded work as a handheld title. DDD I'm fine with being on the 3DS but if they had made it with the intention of it being for 2.8 (I know 2.8 wasn't a thing by 2013 but hear me out), there's a lot more they could've done with DDD in terms of plot.
As much as people complain about KH being convoluted, it's really only DDD that made it that way. And you can tell that in KH3 Nomura was like "oh shit maybe I goofed here", because other than the Power of Waking, Riku being a Master and the ending leading right into KH3, the events in DDD aren't really addressed all that much. Although DDD is probably the way it is because of it setting the groundwork for the next saga, since it's clear that dreams and sleep are a big part here.