You always have a 20 percent chance of randomly guessing the right answer to a question, which is better than not answering the question at all. Moreover there would be a severe penalty for failure to complete the test. According to the GMAT folks: "If you don’t get to all the questions in the verbal and quantitative sections, your score is reduced by the proportion of questions you didn’t answer. So if you fail to answer 5 of the 37 quantitative questions, for example, your raw score would be reduced by 13 percent and your percentile rank may go from the 90th percentile to the 75th percentile. Similarly, if you fail to answer just 5 questions out of the 41 in the verbal section, your score could go from the 91st percentile to the 77th percentile. That’s the kind of score reduction that could make a huge difference to your admissions chances!"