I believe that people who argue like that (and aren't just devil's-advocating for the fun of it, an attitude I find incomprehensible) are themselves unsure, and are seeking validation by trying to convince others. Neither skill in rhetoric, nor force of personality, can confer rightness, but they can seem like a handy substitute. Fortunately, I lack both (possibly all three).

There's a Chesterton quote that fits here, but I can't remember it properly...(and no, I don't mean "What do we do now, Doctor?")