That I don't really agree with. COM basically sets the stage for KH2, if you skip it your left with a few unanswered questions and most of the stuff in KH2 would feel like ass pulls. If you skip over Days the only thing your left wondering is "Who is that black-haired Kairi?"
You're ignoring the fact that your own examples makes your claim redundant.
CoM, when it was released, didn't seem to contribute all that much beyond character development. So Riku got over his Darkness and Sora was restoring his Memories. Fun. If they wanted to they could've had Riku meet up with Sora once Sora woke up and all would've been well.
But they didn't, and
that is where CoM's importance stems from - the games that came
after it.
Judging Days' value now is nothing short of stupid, as much as people might dislike it. BBS's secret ending shows that
Xion is amongst those whose torment need to end, or so I've been told around these parts, and Sora's internal structure was made into a much bigger issue than it originally was with just Roxas merging into him and Namine taking whatever she might've taken from him which was enough of an issue already.
So yes, Days basically gave us Xion as far as anything potentially useful goes, but it
did give us Xion, with ties to Sora, the Keyblade, to Ven, and most likely to Riku.
And it's not until the series ends that we can judge whether or not the game was redundant because the last time I checked, new games didn't
have to contribute too much to the previous games - they needed, if they were side stories, to help set up the
future of the story. So why the heck are you all looking over your shoulders and whining it didn't matter any to KH2?