I probably shouldn't snark so much about the spin-offs, especially since one of them (Birth by Sleep) happens to be my favorite game in the series. Still, KH has become one of those franchises where the creator makes spin-off after spin-off in order to explain/retcon the plot holes of earlier installments.

BBS - Explains the full backstories of Xehanort, the Keyblade War, and even retcons the whole "Chosen Wielder" thing between Sora and Riku (Sora was chosen because he made a connection between his and Ven's heart while Riku was chosen solely because of Terra performing the ceremony on him).

358/2 Days - Explains a more detailed version of Roxas's backstory, and then proceeds to nosedive into bad fanfiction territory (yep, I still hate this game).

Coded - Follows up on the "Thank Namine" plotline from CoM and KH2.

I'm pretty sure 3D tried to explain a few plot holes, but I can't think of them at the moment. Well, that and 3D's story as a whole gets confusing as hell because they decided to throw time travel into the mix. Oh and... seriously, 13 versions of Xehanort? That's the best they could come up with for the 13 Pieces of Darkness plot line? *sigh* Remember when the Disney villains had this Council of Evil thing going on way back when? I sure miss that plot line.