You know, it's not just that good arguments can persuade me. It's also that bad arguments can dissuade me.
There have been plenty of times when I've gotten into conversations about a subject on which I am pretty ambivalent--I don't see a very compelling argument for one side or another. Then a proponent of one side gives me a really poor argument for his side, and that alone makes me feel more inclined to agree with his opposition.
Is that even logical? There's no rule saying one side has to be right. Depending on the situation there could be numerous alternatives besides those presented by two sides, or even if there are only two alternatives, those two alternatives could suck equally but for different reasons. If I haven't been given a good argument for the opposition, why should I think the opposition is correct?
But I do. If I hear enough people say foolish things for their own side, I start to assume (hope?) the other side must be smarter. And I bet a lot of people assume that.