From GRASS:
This becomes a problem because, over time, the plot takes precedence over the characters. The shift creates a transition where, while once the characters drove the plot, now the plot drives the characters. More and more we see dialogue being used up on explaining plot points rather than giving insight into the character who's speaking. Character-wise, the series runs the risk of becoming shallower and it's becoming exceedingly difficult to develop these characters. That's how they develop. Not when they speak about or do just anything, but when they speak and act in such a manner that you can better understand them.
Days missed the perfect opportunity to be more character-centric. We know all the characters and the general plot, so all that was left was the finer details. Instead Nomura chose to throw a new character into the mix to overshadow characters we already know and care about, and add more convolution to the plot (Now Sora's memories are all over the place, as if it wasn't bad enough!).
Some people argue that Days would have been boring without a new character but they're entirely missing the point.
It would have been boring without a new character if the ones we knew had already been fully developed, which is clearly not the case. Wouldn't it have been nice, for once, to have looked at a Kingdom Hearts game and not try to make sense of its plot, but simply just get a better appreciation of its characters?
Days was unnecessary filler one way or another but that doesn't mean it had to automatically be bad. If anything, it could have enriched our perspective on the other games.