The main point of Days was surely to show how Roxas' time in the Organisation played out, meaning what transpired in that year Sora was asleep. It was also one of the four stories Nomura wanted to cover.
It also gave further hints and things to think about regarding hearts and Nobodies, and although many people didn't like Days because the story didn't play out like their headcanon before it was, it is undeniable that Nomura and co. made a severe error by turning Xion into a single-character spotlight-stealing squad, not to mention that each and every story arc in Days was either centered around her or heavily involved her.
Re: Coded is at first glance only important in regard to Mickey's letter and the foreshadowing for DDD in the secret ending and also much shorter storywise than any other KH game, which is due to its original status as a phone game though.
At second glance though Re: Coded has two more important assets that actually foreshadow two of the major reveals and incidents in DDD:
One is the fact that feeling the pain of the tormented ones if handled wrong is dangerous to Sora's heart. Data-Namin? warned both Mickey and Data-Sora about just that, which was exploited by Xemnas, Xigbar and Young Xehanort in DDD in order to have Sora's heart fall to darkness.
The other is the issue that hearts can be born in entities that normally do not have them as Data-Sora grew a heart during Re: Coded despite being a data construct, which actually foreshadows the revelations about hearts in DDD.
All in all, about Re: Coded you can say that while it is certainly the least important out of all entries in the series so far in terms of story development, it is certainly not pointless or a mere rehash as it has elements that complement and reinforce some story points from the other titles.
Not to mention it has the most varied gameplay of all titles.