The idea is that, if you think the reader will just accept something as a fact, you don't need to hedge it. If your reader will see it as an arguable point (in

The idea is that, if you think the reader will just accept something as a fact, you don't need to hedge it. If your reader will see it as an arguable point (instead of an uncontroversial fact), then you should make (or at least sketch) the argument, because you're not a sect leader or anyone else who can spoon-feed "truth" to his minion. You should make the argument because you want the reader to be able to disagree with you and see where he/she still follows you. And if it's not worth making an argument, there's an opportunity to make the text shorter and more concise by exactly that sentence.