You want to make sure you know the Black Letter Law for the exam. You don't want to include only "reasoning" in your answer, such as policy-based reasoning. I was just reading about this in "Getting to Maybe" last night. The authors (experienced professors who have graded thousands of Bluebooks) said that there are often two types of people who give poor answers on exams: 1) The Rule Fanatic (they used different wording), and 2) the "Judge as a Loose Cannon."
The first type of person sees the law is only rules and gets confused if there is an ambiguity in the facts, for example. In contrast, the "Judge as a Loose Cannon" doesn't know the law at all, but tries to use policy and BS to come up with the answer. You don't want to be either of these people. Know the Black Letter Law, but also know how to use it (apply it) to come up with an in-depth analysis under very tight time constraints.