Suppose a change in a new iOS version broke your app, and you wrestled with it for a weekend trying to fix it. How many other developers' weekends could you have saved, if only you were permitted to blog about the issue?
You can't write up examples of stuff using the new APIs, and post it on Github, or share at a local developer group meeting. So inexperienced developers can't learn from others.
It's not like the NDA is protecting things that would be truly damaging if disclosed, considering all it takes for them to pull aside the velvet rope is $100, and they've let both Google and Samsung in. The real secret stuff clearly goes to the blessed developers who get advance notice of things like the taller screen or GPU capabilities.
Edit: not sure why this would be downvoted. Here's a macrumors source on the videos being free. It's also on the WWDC FAQ. http://www.macrumors.com/2012/06/19/apple-posts-wwdc-2012-se...
It is a little silly they keep insisting on an NDA.