I could think of how right minus wrong exams apply. Your right answers will be deducted by your wrong answers. In short, you can end up getting a NEGATIVE SCORE. I remembered how I ended up saying, "Don't ask about my result!" back in high school. Later in College, one exam I took was right minus wrong -- I really just hated the idea of right minus wrong exams. On the other hand, part of me still thinks that they should remain for a good reason.
What good reason would I give a right minus wrong? Well, you can face it how I could use it as a punishment test IF people are caught unprepared. I'd normally think it's unnecessarily cruel except if people have a bad habit of not getting prepared. It should be a negative reinforcement against unpreparedness or misbehavior. While I won't choose it as my method but it'd be good to sometimes enforce it especially against a misbehaving class.