Well it wasn't important enough to be mentioned in any game except 358/2 days.
Roxas wasn't mentioned in KH1, does that mean he's not important?
On topic, I think days will have a concept video for BBS, then BBS will have the video for KH3.
And to agree with some people, Days pretty much looks like just a fan-made game to me. Sure, we get to learn about Roxas and the origin of Org. 13, but as someone already put it, Roxas most likely won't be in KH3. All the Org members are dead. DiZ/AtW is dead. Namine is fused w/ Kairi, and Xion has been wiped off the map. Who else in Days, besides Riku and Mickey, have any importance in KH3? Sure we know Xehanort in some way will have some impact on KH3, but it doesn't directly mean Xemnas will.
When CoM first came out, and even after KH2, there was a huge uproar on how awesome Org.13 was. Then they annouced Days. IMO, its just here to make fans happy, and MAYBE answer some questions that we had during CoM/KH2. As for relevence to KH3 however, I believe it will have very little, if not none.