Ambiguity: Failing to interpret information as the test maker intended can result in a "incorrect" response, even if the taker's response is valid. The term "multiple guess" has been used to describe this scenario. A free response test allows the test taker to make an argument for their viewpoint and potentially receive credit.
No partial credit: Even if a student has some knowledge of a question, they receive no credit for knowing that information if they select the wrong answer. Free response questions may allow a taker to demonstrate their understanding of the subject and receive partial credit.
Logical reasoning: Test takers may be able to rule out answers (due to infeasibility), or even test each answer individually (especially when dealing with mathematics), thereby increasing the chance of providing a correct answer without actually knowing the subject matter. On the other hand, especially on mathematics tests, some answers are included simply for this reason, such as the giving the equation 4x^2 + bx=3 and asking what b^2 equals- it cannot possibly be a negative answer.
The use of multiple choice question in educational fields is sometimes contested due to some of the negative aspects, whether actual or perceived, but the format remains popular due to its utility.