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In the future, lectures may become relevant for more of the reasons that live rock concerts are relevant.
In the past, one chose MIT over other experiences because of reputation.
Perhaps that was once true of rock concerts too ("You gotta see this guitarist play!"), but these days one attends a concert only after one is already a fan. That is, you hear some music, you decide you like it, perhaps you buy some recordings, and eventually you decide you have to shell out for the live concert experience. You get to see your idol in person, and you get the experience of witnessing him or her interpret his or her music in a way that is familiar, but probably slightly different from every other time it's been played.
I see a clear analogy here. In the future, one will likely experience MIT before deciding to attend. This is part of the story of edX. So, lectures may not be solely for the purpose of hearing new ideas for the first time. Rather, they'll also be for the purpose of seeing your idol in person; for hearing the professor deliver a well-known lecture, or lecturing on a topic with which you're familiar, or even just lecturing in a style uniquely his or hers.
Many more people hear the music of (insert your favorite musician here) than see him or her in concert. Many more people will experience MIT remotely than will attend in person. And unlike the concert experience, where one pays a la carte, the MIT experience will presumably remain an all-inclusive blanket price, perhaps more akin to weekend-long concert-a-paloozas than to individual concerts.
But one thing seems certain: when the music (or course content) is available for free, the value of the in-person experience changes: indeed, the reason why it's valuable changes.