The author in this model essay never answers or presents counterarguments to his many rhetorical questions. So why would the author choose to conclude a paragraph and write this essay in the form of questions? By allowing, inviting, and 'opening up' to counterarguments, counterclaims, and rebuttals, don't questions weaken or worsen an argumentative essay?
In asking a question in this manner, the author uses (or tries to use!) rhetorical questions. The author does not expect a response - indeed, such a response is not possible, because the responder has way to insert himself or herself into the piece. These questions are not meant for "allowing, inviting, and 'opening up' to counterarguments." It appears the author's intent is to ask questions that (to the author) have obvious answers in a way to engage the reader more.
Rather than looking at an artificial, bad example, it would be more helpful to look at a canonical good example: the speech given by the Corcyrean Envoy at Sparta prior as told by Thucydides. In the opening of the speech, the envoy effectively makes use of several rhetorical questions.