True or false: In the real world of writing, essays are more interested in effectiveness than in purity of form.

I would agree that this is true. The word "essay" is from French, and it means "to try," so embedded in the idea of an essay is the notion of risk-taking. When you are trying something, often purity of form must be sacrificed. Further, almost all essays attempt, beyond all else, to be persuasive. An author has a point to make and hopes his or her audience will agree with it. Sometimes, in order to make that point, it's necessary to violate purity of form. Some essays, like Carolyn Forche's "The Colonel," cross genres: many people label this a prose poem because of its imaginative imagery. Whether or not Forche broke the rules about purity, she had an important point to make about violence in Central America, and the essay/poem made it effectively.