I attended very few lectures or seminars.
It is simple really. Majority of my lecturers were really boring, and I could go over the lecture topic better through the course readings. There were a few interesting lecturers, where I would always attend their lectures, but for the most part very few of them knew how to make a topic engaging or how to teach properly. Why get out of bed at 8am to attend a boring lecture, with a lecturer I can barely understand, when I can go through the course readings and come away with a better knowledge of the topic? It usually became clear after 3 lectures whether or not they were worth attending.
As for seminars, I considered them tedious. The tutors were either lacklustre, or total raving lunatics. Discussing a topic with a bunch of people who haven't bothered to do the reading, or hold extremely unjustifiable views isn't my idea of a productive endeavour. I'm doing a course where seminar participation counts for 0% of my course, so why waste my time doing pointless group projects with people who never pull their weight? It was a waste of my time, I could having doing research for my upcoming essay instead of wasting my time on things such as group projects. That said I did have two very good seminar tutors, both senior professors, who knew how to make seminars engaging and facilitated meaningful discussion. But for the most part seminar tutors were uninterested post-graduates.
So there you go. I didn't miss them because of a 'too cool for school' attitude, rather because I realised early on that a lot of it was a waste of my time.