Logical Fallacies, Social Truths.

If you aren't very familiar with Logical Fallacies, then as a reference for this post, you may want to look over the Top 20 Logical Fallacies, as found on The Skeptic's Guide To The Universe.

(Edited to add: Or, if you prefer a list with bite, here ya go, courtesy of a brainy lass elsewhere on the 'tubes that... may decide to thump me for calling her that.)

Logical fallacies don't follow the rules of logic. But they remain in constant use, and they repeat often enough for these violations to have names. This is because while they may be bad logical arguments, they're fairly solid and sensible social moves. Let's take a look at those:

Establishing Credibility
It's important in any social situation to establish credibility, including the credentials involved in both sides, where the majority lies, and the like. People take cues from both the many and from the experts, and avoid taking social cues from those on the fringe. In terms of a debate that is supposedly objective, establishing credibility leads to the fallacies: Arguments from Authority, Tu Quoque, and Arguments Ad Populi. Questioning the credibility of others can easily lead to Ad Hominems.

Give Heuristics
Heuristics are fast-and-dirty methods of judgement. "Rules of thumb", without the unfortunate history of that phrase. People don't derive the solutions to problems from basic principles most of the time - we cheat. We find fast solutions that work in a variety of situations, or prioritise simple values; we do this because it works in the kind of messy situations we individually find ourselves in. Such fast-and-dirty rules don't scale to larger or smaller situations all that well, of course. And this leads to further logical fallacies: Confusing Association & Causation, Confusing Unexplained with Inexplicable, Inconsistency, and False Dichotomies.

Expression And Emotion
Clarity, the holy grail of communication, is not an easy thing to achieve. To reach it, we commonly use words, gestures, facial expressions, and a whole gamut of other devices. We move from casual speech to emotional speech, we employ colorful and technical jargon. Making communication really work is an expert affair, and almost everyone you will ever meet is an expert of the practice (whether or not they can explain it). Logical argument is stilted and much-reduced communication, and most experts aren't prepped for it. Using the whole range of emotion and colorful expression in logical debate leads to Arguments from Emotion (which aren't listed in the linked article, oddly; they didn't seem to make their top 20), as well as somes kind of Straw Man and Slippery Slope arguments (though not all).

Manage The Framing
Setting and shifting the terms of a discussion is normal. People are often trying to find a way to get others to see things from their perspective. This often involves sharing heuristics, as above, but also include basic values. This, in turn, can easily create False Continuums and False Dichotomies. From a utilitarian viewpoint, some decisions appear binary and some categories of action are easily dismissed as all bad. From a mystical viewpoint, the same kind of quick categorisation can occur, despite sorting the world into entirely different categories.

...So?

The purpose of this isn't just to point to the idea that normal conversation is irrational or wrongheaded. It's also to show that many of the most common ways that we wander from logic and objectivity are, of themselves, sensible in the context of human life. Sometimes such illogical sense is the right choice, when dealing with other people.