Okay, let’s examine some of the basic flaws in your analogy, then.
First of all, the Straight Dope has thousands and thousands of members. The people who post here are generally unknown to each other - it’s all just lines of text, no faces or real life identities to humanize each other. It’s also a continuous, 24 hour operation, and has been for ten years. And the conversations can play out over the course of weeks, if not months.
All of these are factors that have an influence on how posters behave, that make the social dynamics here drastically different from a dinner party, which has A) usually fewer than a dozen people, B) all of whom are personally known to each other, C) lasts for a few hours at the most, D) which naturally requires that conversations take place in even less time.
So, really, comparing the SDMB to a dinner party is a poor analogy, over all. If you really want a real-world analogy, a better one would probably be a small town. In a small town, Jim and Joe might not really know each other more than in passing. And if they get in a fight one weekend at the local bar, they’re generally not going to start swinging if they bump into each other at the Safeway the following Wednesday.
And the last defense against the idea that the Pit is ridiculous is the practical: it works. A message board that was created, at least in part, as a place to discuss contentions issues like politics and religion is naturally going to get heated - people frequently kill each other over things like this IRL, after all. Even with a strictly moderated “no fighting” rule, people are going to get pissed at each other, and they’re going to want to communicate just how pissed they are. Before we had the Pit, they’d do this by getting as close to the line as they could without crossing it. Problem there is, the other guy would respond by doing the same thing, and the first guy would retaliate, and eventually you’d get to the point where one or both of them are way over the line, and we have to start handing out warnings or even bannings, and we generally don’t want to ban people if we can’t help it: the board is strengthened by having a wide diversity of posters, and the quality of information available here is hurt if the most passionate posters on a given subject (and thus, the most likely to get angry in a conversation about said subject) keep getting banned because they lose their tempers.
Adding the Pit created a much needed forum where people could blow off steam without fucking up their posting privileges. It’s not a perfect solution: you still see a lot of this, “How close to the line can I get?” behavior in other forums, but from talking to people who were around before the Pit was created, it’s clear that this happens at a vastly reduced rate than it used to, and when it starts up, instead of having to quash it and expect everyone to just swallow their emotions, they’ve got an alternative venue where they can fully express themselves.
You may find the behavior that leads to Pit threads to be ridiculous: I would maintain that it’s simply human, but I suppose the argument could be made that there’s really no difference between the two. But the Pit itself, as a forum, is not ridiculous. It’s an entirely practical compromise to maintain the standards of the rest of the board, and in that function, it’s largely effective.