Understanding people you agree with is difficult. Understanding people you disagree with is even harder. When you comment on someone’s position – especially to disagree – it helps to use an exact quote and then directly engage with their words. The quote should have a source, too, so that people can check the context and accuracy of the quote.
If you specifically attribute an idea to a person, then you should quote it. If you only paraphrase from memory, you may do it wrong, and there’s no reasonable way to refute your mistake. Without a source, no one can point out your misreadings, nor can they see that you’re right and change their mind. All people can say is “uhh i don’t think i said that, i don’t know what you’re talking about”. That’s not productive.
You should use quotes and sources when the person might not be happy to agree that they said something, or when you’re saying something critical or negative.
If the person said something similar to what you remember, the difference may matter. Let’s see the actual quote. Maybe they were precise with their wording in ways you don’t even think about. Or not. We need the quote in order to analyze and decide.
Also, don’t use controversial examples from past discussions without quotes or references. It’s not much of an example if I can’t look at it! People commonly say things like “Joe was [bad thing] in a discussion 3 months ago” without quotes or details. (Examples of bad things to go in that sentence: mean, rude, dishonest, unreasonable, incorrect, wrong.) Sometimes people don’t even give a paraphrase or summary, they just claim something happened.
Often, people didn’t criticize Joe’s statements at the time they were said. Now they are bringing them up without any details. This avoids analysis, at the time or later, of whether their claim about Joe’s statements is correct or incorrect (that typically seems to be the goal of not using quotes). And it indicates they were holding an unstated grudge, which was hidden from criticism (like correcting a misunderstanding or incorrect logical reasoning). They never gave Joe the opportunity to change his mind, retract his statement, learn from his error, or refute the charges – and yet they remembered it negatively, or else they wouldn’t have brought it up negatively at a later time (especially without a quote, which means they didn’t go look it up to refresh their memory). It’s also especially unfair to expect other people to remember something that you thought was negative but you didn’t complain about at the time – you didn’t draw attention to it, so why would others have picked out that particular thing to remember?
Rational criticism involves explaining why something is a mistake. It has to be possible to learn from the criticism, but Joe won’t learn from being told an unspecified past statement was bad. And it has to be possible to refute the criticism, but there’s no way to give counter-arguments when the details are missing. (All one can do is refute the method of criticism for not using quotes, but that doesn’t actually mean Joe didn’t do the bad thing.)
So, at my forums – and I’d recommend this everywhere – don’t make unsourced accusations.