Look, you may think that Kid A is a better album. But the fact is that OK Computer is like Dark Side of the Moon in that's more well known and debated. And that's the key to me, so many people I talk to who have no clue who bands like Arcade Fire and Pavement are actually know Radiohead.
I would argue that Wish You Were Here or even Piper At The Gates of Dawn are better than Dark Side of the Moon, but if somebody ever asked me where to start with Pink Floyd, I'd say Dark Side every time. Same with Radiohead. They're great albums on their own, but they also make for convenient starting points in their respective discographies as well as litmus tests for whether or not you'll like the band.
To speak a bit to what Khar was saying, a generational touchstone doesn't have to touch every single person in the world, not to mention in a good way. Hell, I routinely meet people who either haven't heard or don't like the Beatles, and I would consider them one of, if not the, most important bands ever.
