To begin with, I don't hate Kairi for being useless. I can deal with that, and for what it's worth, she is somewhat important albeit in a passive manner. That's a somewhat 'objective' flaw to her character I can forgive her for.
Why I hate her is because she's obsessive compulsive towards Sora and a bitch towards Riku based on numerous things she said which can be taken in certain ways that tie in together.
As for liking the characters that look like her (more general definition so I can throw Aqua in as well), it's like they realize how badly they failed with Kairi herself so they're trying to make up for it by everyone they can and relate her to them to supposedly lessen the blow.
Hence why Namine is amazing, and both Xion and Aqua at least seem to be promising.