As a big geek, I've watched and loved all of those movies. I love good CGI (hell, I love BAD CGI). I love watching giant spaceships and dragons and eldritch horrors from beyond. CGI is definitely an effective way of achieving those visions and can create some amazing, jaw-dropping experiences. Sometimes there is definitely no way to replicate those visions with practical effects.
But:
At the end of the day, that isn't the story, that's the vision. A good story is a good story if you went all out on CGI or if you filmed shadow puppets. I can't think of a single story that couldn't be told without the particular visuals that were used. Absolutely the creature in the Abyss would be imposible to do without computers. But could you tell the story without showing the creature? Of course you could (and it's a great film, so it would hold up). Avatar was jaw droppingly beautiful...could it have been without computers? Of course (hell, I've had jaw-dropped reactions to well done matte paintings, let alone expensive flyover shots).
Again, I love CGI. I think it's a wonderful tool that can create a cinematic experience unlike anything else. But it's a color. You can tell a wonderful story without green. I think that is the contention that is made when saying "CGI is unnecessary". Well, that and a bit of envy (myself definitely included here)!
Anyway, Jurassic Park blew my mind. Sin City was a blast, and wonderful. Final Fantasy: The Spirits Within I would have enjoyed if the words "final fantasy" weren't in the title (though wise for a mainstream release. I loved "Advent Children", but most who didn't play FF7 were lost). There was great CGI in Lord of the Rings, and likely will be in the Hobbit as well. Hellboy 2 stands in my mind recently. And yeah, Tron, Last Starfighter, Lawnmower Man...beautiful stuff! Oh, and we're mentioning animation, both Up and Despicable Me were really great.