It is the main story. Literally all KH games, numbered or not are part of the main story. 

Hm, who can Luxord be

I disagree. I know what they're about, I just think that they involve sideplots that tie into the main struggle.

Dream Drop Distance isn't a sideplot, it is imperative to the continued story that it moves forward.

Holy hell, dude, stop commenting on a series you clearly haven't played (or at very least didn't pay attention to).

True, and the series is worse for it.

I played all the games, and it became very apparent to me after beating KH3 that it was very unnecessarily stretched out with Shamalan level twists to keep us reeling and ultimately couldn't come to a good conclusion after so many convoluted plot threads. The series story was still strong by Birth By Sleep, but the cell phone games and DDD is where they screwed the pooch in plot lines. I don't think I have ever been more dissatisfied with a conclusion of what started as an epic tale. This includes Game of Thrones.

Look dude, I define it one way, you define it another. The fact of the matter is, I don't give a shit whether DDD is a spinoff or not. I don't care if any of them are. I just hate the way they did this. I love the series, but damn they could have done this better.

Okay, so you're saying it's a spin-off because you think a major part of the story isn't major? Because nothing else about DDD says it's a spin-off aside from naming convention.

And I question how much you love the series, considering you don't even know how important BBS or DDD are to the plot.

I don't get why you still care dude. You just gotta be right on the internet, huh? I'm not changing my mind, and I love Kingdom Hearts, and you can't take that from me.

You love Kingdom Hearts and you insist the unnumbered titles are spin-offs because they aren't part of the main narrative. I'm sorry, but any KH fan anywhere will tell you BBS, DDD, and CoM are integral chapters of the story. And beyond that, there are fans who will argue the importance of Days and Coded. And what about Union X? It's literally setting the stage for KH4.

Your qualifiers for spin-off don't match the reality of the games you're talking about. It'd be one thing if you wanted to be obtusely conservative in what a spin-off is and just say "anything unnumbered is a spin-off". At least you would be logically consistent. But you literally have no ground to stand on based on your own definition of what a spin-off is to say Dream Drop Distance is one, the only ground you have to stand on with Birth By Sleep is you don't play the as Sora (despite other non-spin-off games in other series featuring new characters and the fact that Sora still plays an important role of Ventus' story), and the only argument you have for Chain of Memories is the combat (though a more hardcore KH fan would point out the heart of CoM's combat, that being stocking abilities with a cool down, seeped into the rest of the series and negates the qualifier). Even Days evolved the combat a bit while shedding light on a semi-important character in KH3 that played a huge role in Lea's character arc. 

The only unnumbered title I would near-completely relinquish as a spin-off is ReCoded due to the fact that the story amounts to "And then Mickey remembered BBS happened", but even that evolved the combat a bit and set the stage of DDD.