Where you are trying to convince someone, your role is similar to a lawyer. You have to convince the reader by telling a particular side of the story. But when you are writing an informative essay, you have to present both sides.
Some writers don't need to show the point of view of the other side. For example, if you are writing for the rights of women, you don't need to show why people thought that inequality of sexes was an act of divine will.
But, I think, it depends on the topic you are writing about and your emotional state at that particular time.