I don't know would it be of any use of you but I believe my country has an unique experience with educational systems based on both models-e.g. mandatory lectures and non-mandatory ones. About 30 years ago my country was part of the communist East European states and the lectures in the universities were mandatory. There was a special clause in the university codes requiring students to attend their lectures otherwise they could get expelled very quickly. You should know that then university education was a kind of dream for most of the people and students went to dormitories away from friends and families to study. Also, their expenses (for the most part) were covered by the state, so university student was "social position", not mere education and getting expelled could have ramifications beyond the university. Then even missing one lecture could be big deal if the professor decided to punish the student. This is why students went to lectures without caring what exactly the lecturer was saying and even what this class is all about. Our experience is that mandatory lectures always (I tell you this is for our country in particular, so I can't say it's universally true) led to lower interest in the subject, lower grades and even bribes. Yes, it was a common practice for someone to use his or hers "connections" (as we call them) to try to get some rare foreign goods for the university professor, give him/her money or even just some spirit drinks ( I have heard of such cases too, not joking) to manage to pass the exam regardless of what the student knows. Take into account the fact this was when lectures were mandatory and everyone was attending them. Can you imagine how low was the interest then to have people resorting to bribes instead of just listening to what the professor says?
Then, things changed and the system got progressively freer. Some universities tried to hold onto the "old ways" and punish students for not attending lectures but as the "exclusiveness" of university education got less and less privileges they started to understand there wasn't much effect of punishment. I still know of one university trying to impose fear on its students for not attending lectures, but the general trend is they don't care any more who goes and who doesn't. The reason is that when the borders opened many people fled the country and some started to bring foreign experience in to our (then outdated) education systems, some started to demand more rights and freedom and made the case no one should force anybody to attend anything and then some just made money but had no education and in order to get the social position of being educated, not the actual skills and knowledge they simply bought their diplomas. It was (and to some extent still is) big business in my country. But the results of these practices started showing in ever lower and lower quality of education and more and more "bought", rather than "deserved" degrees. I'm part of this generation and remember absolutely free classrooms where out of a nearly 150 students by names you had only 5 real people in the room! I think you can imagine what the attendance rate was. But universities kept their lectures (sometimes only because the professors would otherwise lose their jobs and the unemployment rate was very high so they pledged their colleagues in the administration to keep the lectures) and now the students themselves are returning to the classrooms. My country had some really rough times the last 20 years but now it seems like the people themselves, not the authorities, the social position or even the economy are forcing them back in the universities. Now the clessrooms are again getting filled because some people themselves chose it is the better way to learn, than by themselves. Now, I could say our country's education system is as free as it can be-nobody forces anybody to attend anything, the professors can chose their curriculum, anybody can read anything and hold whatever opinion he or she thinks is right. The value of higher education has greatly diminished over time and those who have money even don't want to buy their diplomas any more (not because they can't, but because they don't see the point in having an education when you have money) but the people still return to the universities and now it's the people themselves who demand lectures. Now, I can see people in their 30s and 40s going back to school and attending lectures not because anybody demands them to, but because they simply want to, because they feel the need to know more and going to lecture and having personal contact with the lecturer is their own way of getting to know the subject, too. I can see now how my generation is starting to wake up for the value of education and especially this coming from actually knowing what somebody thinks about this or that subject and how s/he can teach it. I can say now people actually start to think of universities more like places where they can go, meet somebody who has knowledge and use his or hers skills to educate themselves or get an answer to specific question they may have. And having a good lecture course becomes essential to manage to satisfy this need. Now the universities are becoming ever more social rather than mere educational institutions and anybody can open university if s/he has the cash to do it. But people don't go to universities for the diploma, they go there to know something they don't and require the professors there to have the knowledge they need or simply desire. This is what a good lecture course is about (at least in my mind-you're free to disagree).
But you may wonder why I'm telling you all this @Troway Jestman. The reason is because I believe you aren't very old (please, forgive me for the personal intrusion here) and probably because you have only your country as an example (which I believe is a western country). But this isn't the only possible example. See, my country has been through both periods when lectures were mandatory and those who attended them had a higher social status than those who didn't and periods where almost nobody went to lectures and people were thinking about actually abolishing them. But in my mind both these alternatives are wrong! And I'm not talking here from some subjective theoretical point of view-I'm talking from personal(and national) experience. When higher education was seen as some "special privilege" to the society's elite and at the same time lectures were mandatory for students and the common people were excluded from attending them, the quality was poor and people didn't learn much from them. So you can see that even when you give incentives to people learning from lectures and at the same time make them mandatory it turns against you and you actually lower the quality of education, rather than increase it. On the other hand when you make everything free, yes, the attendance rate drops and almost no one cares to come but those who come are there for a reason and the dialogue and in general the very quality of education is increased when you have a lecture course parallel to the books and/or internet video course about the subject. And it also makes the professor him/herself do better job at assessing his field, educating him/herself on the novelties there and in general providing better services for society, not just for education. I can go on and tell you many things terribly wrong with my country and its education system but from the perspective of someone who has seen both the effects of mandatory lectures and lectures with almost no attendance at all I can tell you it's better to keep the lecture courses in, rather then abolish them-it's better both for professors and students and for the society in general. And this isn't an answer based on general knowledge but on personal observations. I firmly believe good lecture courses are a must for any modern university, if not for its educational role, than for its social role. Lecture courses can "filter out" those students who really care about some field, provide the professor with the environment to shape his/her course better and improve his/hers communication skills and in general provide the ability to create an environment for learning not only for students, but for everyone interested in this particular field.
I believe this is the true complete answer of your question :)