I am confused on what you mean about students taking a comprehensive exam in order to graduate from their major. Students already do this by learning continuously for 4 years. They learn the basics of their major and then they learn into a specific field. Each class that they take overtime is continuously building on their past information so they need to know everything they learned in the past in order to do what they are doing now.
I know the university I attended did not require comprehensive exams to receive a major. Rather, you had to take a 3 credit class for your final project, more than likely a paper, I assume. I would much prefer a comprehensive exam however, so as a student, I'm not wasting my time putting together an extensive paper for no reason at all, other than to receive three more credits. The exam seems easier and less complicated.
Me being a college student myself, I am unfortunate for this idea. Students are required to take certain amount and level of classes, and if they are able to receive their degrees without taking such upper level classes, they could very well miss out on important information regarding their job field.
Yes, students should not receive their degrees until they pass comprehensive exams in their majors, because it is not enough to learn the material for a short period of time, or to show up passively and not learn at all. Students need to demonstrate that they have retained the material, and that they know something about the subject in which they are being granted a major.
In order for students to get their degrees, they must pass certain tests like everyone else. A comprehensive exam proves that everyone gets the same education for certain degrees. These exams also ensure that students learned things before receiving degrees to certified they graduated. Exams are another way to see taxpayer money is going to the right place in state-supported schools.
When taking studies in college, students are already given plenty of exams in their majors. Hopefully, they also have advisers who will let them know--based on grades and skill--if they're not headed down the proper path in choosing a major. This knowledge would especially be important for anyone who is going on to another level of schooling. However, a comprehensive exam shouldn't be necessary since the student will already have concentrated on that topic, and will have had many exams along the way.
A degree program generally has pre-requisites in order to finish the degree. Most often these do not include comprehension tests. It would be unfair to require comprehension tests when they have not been required for prior degree programs. Completing the degree requirements should be enough to get your degree without further testing.