Personally, I do believe that every single class should require mandatory attendance from students. Although nowadays there are a lot of hi-tech technologies are involved in our colleges and universities where every student can "attend" a class without leaving his/her house, listen to presentations online, chat with other students online and other various stuff, but I think it's not a very efficient way to gain proper knowledge. Moreover, one of my friends, who's an A-grade student, almost failed her online course because the whole materials of the course weren't covered completely. That's why I prefer attending class on a regular basis, i.e. going to college and study there.
I think there are not any disadvantages about attending a class. During a class I take notes about material, watch and listen to various important examples from my professor, participate in discussions, analyze different problems and find solutions for them. I'm sure that skipping a class is not a good idea. Skipping classes shows a student's lack of interest and seriousness towards a class, shows disrespect to a professor. And all of these issues will cause a negative effect on bad grades on tests.
In my opinion, attending classes is important to me because some of the course materials are not covered in books. A lot of professors are text-oriented, but there also ones who teach their own way. Instead of reading and teaching from books, those professors provide various presentations on the material, show experiments, tell stories relating to the material. Shortly, those professors try to make a class interesting. Further, those professors give tests based on their own lectures, and not the books, so I'm sure it's better to go to such classes.
I think that classroom class coverage present information in a different way than the books, and that's why I'm an ardent supporter of mandatory class attendance. But I know there are reasons that can prevent...