But you have an identity mechanism; even if it is per thread, it allows you to attribute a collection of writings to a single author.
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.