There are three reasons I think one should attend classes:
1. Regularity: How much ever one may try to be regular, it is really hard to do that on one's own. It is much easier to regularly attend classes in person.
2. Ability to ask questions: My philosophy of attending/teaching a class: "There is no point of a class if you cannot ask questions." A teacher can change his teaching style/content based on the understanding of students in the class, which is not possible in a recorded lecture. If the students are unable to grasp a concept, the teacher can go slow or he can go fast if the students find some concepts trivial. Again, this is not possible in a recorded lecture.
3. Relationship: Relationship with teacher makes things easier to understand. If you like a teacher as a person, you will enjoy his/her classes more than a person you hate/don't know. Moreover, in the future if you need a teacher to vouch for you, e.g., through a recommendation letter, he will not do that if he has never ever seen you. Definitely, the NPTEL lecturer is not going to write you a recommendation letter.
Even if 1. was possible, 2. and 3. are impossible through video lectures, therefore one should attend classes. Moreover they are fun, you get to see other human beings. Better than sitting in the hostel room in front of the computer.
I like to blog about various topics ranging from education in IITs to social issues in India and US. All opinions and views presented here are my own and not of my employer.