As for attendance, most of my lecturers just read out the notes. If they have nothing further to add then sometimes (not always) I don't bother to go as I will sit there clock watching, drawing pictures or filling in a sudoko / crossword. If I have a lecturer who adds further value to the notes and who I find interesting I will attend and pay attention.
I know a few people who missed all the lectures but did a past paper every week and aced the exam. Lectures generally don't mean much, it's tutorials / labs you need to attend. Lecturers usually give a certain number you must attend or fail, although this is not always stuck to. It's there to scare you into going.
The only other point is in exams, if you lecturer knows you attended the classes and can see you tried to answer a question but found it difficult they may give you partial marks. If they never seen you attend they may just think 'this kid is just talking crap trying to BS through exam' although our exams are sealed so names are not shown when marking.