There are two points of view to this question. Some people believe that students are adults and they have their own responsibilities. So, they should have freedom in arranging their time. Other people think that optional attendance has a corruption effect on the educational system as a whole. Personally, I think that this question is more complicated.

First of all, many students have already children. So, they sometimes have to skip a class or two in order to perform their responsibilities. Second of all, many students do not have enough money for their needs and they have to work more than other students. In this case, optional attendance is well-taken.
Another important aspect of this subject that sometimes lectures of a particular teacher may be dry and uninteresting. I believe that it is reasonably to skip those classes and learn all by one's self. It can save time, so a person can spend it on a research or preparation for a coming test.

From the other side, I think that some students who does not have any responsibilities may skip classes without any reason. They can have more time to attend clubs and parties. It has a negative affect on their grades. So, it can result in a waste of money and time. Students may take the same classes more than one time to meet the requirements of their program. It is a waste of money for both a government and a family. A government will lose money because they are spent on education and it means they are tax free. At the same time, parents will have to pay for education of their child one more time. Another important aspect of this that students will not learn how to arrange their time in order to have all things done on time.

To summarize, I think that students must attend classes. However, some students with children or who have excellent grades and already work in the field of their major should be allowed to skip a particular amount of classes.