Just because you aren't necessarily playing AS a character doesn't mean their moment is ruined. Cutscenes go a long way with characters; fan favorite Axel has been playable in one game and relegated to cinematics in all others.
I'd say there's a gigantic *despite* hidden there.
But I can just twist this around: just because a non-playable character won't necessarily suck or feel useless doesn't mean you can't try to make them even more part of the action, nor that doing so will ruin them or the game.
Fandom-hated Kairi has been relegated to cardboard cutout since Chain of Memories and she has as many enemies as fans (if not more) and she embodies the definition of useless in many people's minds.
And yes, I agree you can make her better also by just writing her better, but you gave her a weapon, a combat weapon. One that very important people wield in the game where those very same vips are going to duke it out in a big clash. I'd say this makes her case a tad different than Axel's development throughout the series and justifies her fighting an enemy or two.
End game bosses should test the skills you've practiced, not ditching them for completely new ones. Having Sora and Kairi team up against a Nort goes way further to benefit story and gameplay. A 2v1 where Sora and Kairi sync up to use boss specific limits is way more hype than putting the main character on the sidelines and making the boss essentially a set difficulty gimmick fight.
Oh, I seriously doubt this would go full circle and put Sora in the sidelines, nor it should. Have Sora do something throughout the entire story (and that's pretty much a given just by having the game unfold), and at the same time have other people doing equally important stuff. Just the supposed darknesses are thirteen so almost double of the supposed lights, I'm sure we can have different fights without the good guys going on coffee breaks.
I don't think the entire concept/experience of endgame bosses will be ditched if one or two are "sacrificed" for storytelling: Kairi fights Larxene at the very end in a show-off fight, and that's a missed chance for developing your Sora techs? Just rummage through the Barrel-O-Norts and pick another one out of the eleven dudes to give Sora that testing skills fight.
KH2 did this perfectly in it's final battles. Fighting with Riku as a partner is 1000x cooler than just becoming Riku. Same goes for teaming up with Axel, all the FF characters... Seeing how they can assist SORA, the main character, is much better than taking everything away.
That's, like, your opinion man.
Personally I'm a bit sick and tired of people doing stuff for Sora, to Sora and out of Sora. Ally Riku didn't give me any hype after the first 5 minutes of having a Keyblade-wielding ally, and in the subsequent 11 years of playthroughs I so wished I could control such wielder.
Also man you phrased that like the other characters should be honored to even be of use to Sora, and if they dare taking something for themselves they're putting his main character privileges at risk.
Personally I find the "oh no, weird cards/a strong gust of wind/Xemnas magic/a building is preventing us from taking action, it's all on you Sora" in KH II kinda silly and some old trope we should move away from. I understand that in KH II times the brief Riku moments were the best you could offer without destroying the gameplay experience, but we have way better tools nowadays and I think it's time to have stories about friendship actually mean the message they bring and not a glorification of the perfect fearless main character.
This just made me think on Riku's new Keyblade. A big spank paddle like that would have a completely different fighting style from what he's used to.
It's always nice that Fudge reminds us what's really important in life.
Riku will spank all dem youngsters
I demand his transformation should be called "That's a Paddlin'"
EDIT:
KH II was the worst in the aspect of highlighting the other character's stories and experiences by delegating more than 80% of them behind the scenes, a thing that gets painfully blatant when you delve into the KH 2 novels once.
Having characters represented in cutscenes and more focus shifts (like DDD did between worlds) is certainly a step up on this, but there's also the factor of Sora feeling ridiculously overpowered and being given every available agency (from the audience's view) if it is always solely him playable.
Like i.e. Ansem SoD is a sort of Riku's personal enemy/nemesis, Master Xehanort is Terra's and/or Mickeys, Vanitas is Ventus' and so on.
It just doesn't have the same impact nor the narrative and emotional connection if it is just Sora fighting them in a sort of proxy role.
This too. Again, I think narrative impact should get as much loving care as the gameplay, and that's just not happening in the supposed story-ender KH III if it's just the "Sora&pals who are kinda there" show.