Not true. We do not need to accumulate reputation to have a meaningful conversation. For example, I've seen informative conversations on Secret with the only identity mechanism being the icon they're assigned (presumably at random per thread).
Edit: I also don't recall usenet having a mechanism that 'kept score'. Slightly before my time so I may be wrong about this.
I'm not talking about "keeping score" in a system. I'm talking about the ways human handle identity and associate properties with objects. I may not see your karma, but I see your nick. I don't know your name, or anything about your life, but I know your nick is a reference a person somewhere. If I keep seeing comments written by user amirmc that stand out because of their clear argumentation and reasonable tone, I start to recognize the author and accumulate a feeling of respect. So the next time user amirmc disagrees with me, I treat the arguments with little more care and consideration than usual, because I recognize author as a reasonable person.
This is a natural mechanism of human interaction. My point is that if we reject the entire concept of identity, we will lose a lot of information vital to evaluating an argument.
Within forum post culture there's a built-in tendency for content to be fast and disposable. This makes identity relatively more valuable, because it allows a stream of posts to be put in context, just like with a book.
Yes, they probably can't find out that the 10k-karma-guy is Jack Miller, but all readers of his posts will be influenced by the 10k-karma.
HN hides the karma mostly. But SO shoves it right in your face, that some gold-20k-guy wrote an answer.
Also people will vote stuff up because they like it. This may be because it's high quality, but it doesn't have to.