There will be two types of leaks for the most part.
The negative leak, people who hate what they see, don't want it to happen and bad mouth it because they have nothing to lose because they don't want to part of the new edition. These will be made up of folks that hate WotC, have an axe to grind, the eternal edition warriors, and so on.
Then there is what I call the "blabbermouth" leak, the guy or gal who is so damn excited they just can stay quiet about it. Its like being around Mike Mearls when he starts talking about the game (I speak from personal experience, when I was last at D&D XP Mike loves to talk about D&D, I had a long chat with him in hall one night). These folks just cannot help themselves.
The NDA can be used on these guys but its not really that effective.
But, it's not just about leaks from the fans...
There is another class of player at D&D XP. They are the guy and gal that works for another game company, freelance for other game companies, writes for a retro-clone, dreams about making their own RPG, etc.
Here is where the NDA saves your bacon. If someone turns around and incorporates elements of D&D Next into their product or work from what they see at D&D XP, WotC has legal grounds to go after them. As it should be. They are putting huge resources into this, they don't want to lose or lessen the benefit from doing it.
So, why not just ban those guys from the tables? Not really a smart thing to do, and do you know everyone that is going to work on something in the game industry? If WotC is serious about bringing the community together and having 3rd party support for their new game, then other game companies and their employees/freelancers will need to be part of the process. Its not just players that will make D&D Next successful. You DO NOT let your potential competition see the goods without some protections in place.
Again, you have a choice. You can sign the NDA and go see what the fuss is about or you can not sign the NDA and go play D&D 4e to your heart's content at D&D XP. Dem der da rules. Are you in, or are you out?
My Two Coppers,