Rick, I was just looking for answers to these questions, and had only moments before finding your comment attempted making a cloze question that delivered a large essay box by putting a 0% answer using lorem ipsum. So if a student HAD put lorem ipsum in there, they would've gotten zero credit, solving your problem!
To directly answer your credit, though, I can think of many reasons why you would ask students to complete essay style questions that would be graded on a pass/fail or complete/incomplete basis rather than based upon an in-depth reading of their response. In this instance, rather than the system forcing the teacher or TA to manually grade every submission as complete or incomplete, it gives the students the benefit of the doubt (meaning "something in the box" is assumed to be equivalent to "complete"). The teacher would still have the ability to check the student's work using the same manual grading procedure that you would use anyway, to keep an eye out for the exact behavior that you speak of. But the system doesn't assume every student or even a majority of students is trying to game the system.
As an example, I worked with a class that asked a series of 3-5 "anticipatory questions," which are designed to get the students to think about the material they're ABOUT to cover using terminology or concepts that have ALREADY learned. They're generally essay questions, where we want the student to write maybe 4 or 5 sentences. The instructor, it turned out, was exceptionally lax in reviewing the student's work one semester, but the next instructor the next semester went back and DID review all of the student's responses. And guess what? Out of 30 students, not one of them tried to just fill in the box with junk.
In this case, by the way, we were using the Questionnaire plugin, not the Quiz, for exactly the reason that you can make a Questionnaire 'graded,' set completion to 'must receive a grade for it to be complete.' When you do that, any submission at all gets a "100%." This allows us to track that the students did complete the questions with SOMETHING, effectively "auto-grading" the essay questions. But that's Questionnaire, not the Quiz...
Last, I would love a third activity completion option besides "students manually check" and "must receive grade" of "students must submit something." But that's neither here nor there.