The principle of charity says that you should read your opponent's arguments as if they are reasonable people arguing in good faith. Why is the principle of charity a good thing?
Chris Hallquist asked this question many months ago, and proposed that the principle of charity isn't necessary if we're talking about someone who is dead. (Also see the followup post.) The dead don't care, and sometimes people really are being unreasonable or arguing in bad faith. We should try to assess how reasonable an argument is rather than prejudging that they are probably reasonable.
I agree that it's great to be realistic about how reasonable or unreasonable people are, and not adopt the rose-colored glasses of charity. However, I think there are lots of justifications for the principle of charity that might supersede this.
1. We seem to be biased to be uncharitable. We tend to think of our own views as having such high coherence, and think other people are far less coherent. But we can't all be right. If we want to make the most accurate assessment of how reasonable other people are, we should apply a principle of charity in order to offset our prior biases.
2. It is easy to go from charitable to uncharitable, but not so easy to go back. Once we've read a person uncharitably, it can sour the discussion, and hurt the relationship that was the basis for that discussion. This is basically the reason that Chris Hallquist offered, and he is correct in saying that it does not apply to dead people.
3. If you act as if your opponents are making the best argument they could have been making, then you often make the most persuasive case. If there are spectators to the argument, the spectators might not agree precisely with what your opponent says, but may agree with a stronger argument. So you might as well argue in a way that persuades both your opponent and this hypothetical spectator.
4. The principle of charity encodes our prior belief that someone is being reasonable. For example, I respect Chris Hallquist enough that it seems natural to presume that any post he writes is something reasonable. You might say that this means I'm "charitable" to Hallquist. Obviously, this would only apply to people I trust.
5. Some arguments are adversarial, but some are cooperative. Sometimes it's about getting closer to truth, and not about any particular opinion your opponent holds. Charity is appropriate here, because even if the other person really isn't reasonable, it may raise the discussion to a more advanced level. This makes the discussion more useful to you (though if the other person is really unreasonable, maybe it won't be useful anyway).