Yes, there are countless usenet arguments where sockpuppets got exposed, so we ought to be able to figure out what happens. It looks to me that they make it difficult for the puppeteer to be taken seriously in that venue in the future, and thus are a net loss, but they don't look to me like they damage the puppeteer's side in that venue or the puppeteer in other venues. Sock puppets probably mainly appear in situations that are already so polarized that nothing is really going to happen. The puppetry just gives an excuse to declare beyond the pale someone who wasn't really being engaged anyhow. That's certainly how climategate looks to me: people who don't talk to each other looking for excuses not to talk to each other.