There are quite a few crucial plot reveals in these games, and it fills us in on a lot of other things.
They may not seem like they're worth any time, but they most definitely are.
And BBS is KH0, very important.

BBS is was extremely important due to the much needed backstory.

^EXACTLY!!!

yeah but what im saying is could they have like continued the story without knowing bout this stuff ...great now my heads spinning

^that would've prob made the story worse IMO

^Yeah, too many questions would've gone un-answered, like how Sora got the Keyblade, where Ansem and Terranort, and all of these other people, like the Org. XIII members came from, or WHY KH is so desired...it would be so much more frantic without them. O_o Especially when I start thinking about it...

If they didn't then most the story wouldnt make sense because the 3rd game is supposed to be about Sora saving Ven Aqua Terra Axel Roxas and Xion and you wouldn't know them plus if you didn't play coded you wouldn't know that that's what KH3 is supposedly about. Those games need to be included

well sora got the keyblade off destiny island with riku going in darkness and if sora helps xion woulnd't he go back to sleep again cause she is part of him

Oh, come on...
Birth By Sleep is Kingdom Hearts 0. It is the START. The BEGINNING. You can't jump into the middle of a book and know what the hell is going on. KH1 and KH2 made sense without knowing the full story, but in order to get to the end of the story you have to know what started it.

Xion was made from a separate Nobody experiment by Vexen. She may have Sora's memories but she is not really a part of him.

Riku was the destined Keyblader, Terra decided that in BBS. Now, when Riku was taunted by Ansem in KH1, the power fell into Sora's hands for the time being.

358/2 Days explains further the agenda. personality, and motives of Organization XIII. It also lets you know missing characters crucial to the plot line.

Coded paves the way for KH3 directly, it alone links all of the KH games into one goal for Sora. It gives the plot for KH3 itself. It also has the teaser for Dream Drop Distance, which will give Riku and Sora the power to complete KH3.

Without these games, it would be impossible to even guess at what's going on in the next games. Without them, we'd all be lost. We'd all STILL be trying to figure out who any of the big characters in KH2 are. After almost SIX YEARS of it being out. Six years, of nothing, then jump into the next one with a completely different plot line? Oh yeah, that would work out just fine. Not.

one word to solve everything "ok" not trying to be rude but how long did it take you to make that comment Roxas 1314

It lays the correct path of the plotline. Recoded, BBS, and Days was needed.

About five minutes.
I've explained KH plots to so many people, it's like reciting the ABC's.

@Kadaj, Yes. Plot is key to the whole thing.

well cant argue with that but now that i think about it just in my opinion we didn't really need to see 358/2days

well i actually now only think 358/2days wasn't that much needed

Without Days we would still be lacking characters and details about Kingdom Hearts itself.

All three were important. But Re: Coded was probably the least important out of these three.

Coded let Data Sora tell Sora about the hurt via Mickey, without that...DDD would be a WTF?! moment for everybody including Sora.