I don't really disagree with you. I was just theorizing about a leftist with a core set of values who could honestly debate a topic, from his leftist perspective. I've rarely ever actually run into such a leftist who could do this.
Well, with one exception. An older British guy, Martin Willett, had a forum quite some time ago called "Debate Unlimited". He was an atheist and somewhat left in his ideology. But, he at least had a core set of values and was perhaps the most consistent "liberal-socialist" that I've debated.