Uhm... wut? The GSL is being released publicly on June 6th, and book publishers already have an advance copy. WotC put out a press release & everything.
As for the benefits of 3rd party support, I'm imagine that googling "Ryan Dancey" and "network" will turn up something. Sure, 4e would work fine as a "closed" system. But that's inefficient - there's a certain amount of overheard that goes into making any product at WotC, which makes some kinds of products (notably, adventures) not very profitable for them. Wizards does not have an infinite number of designers, playtesters, nor time, so they will focus on the products that give them the most return for their investment, and the 3rd party support can handle the other stuff. (in an ideal world)
That's without even getting into Dancey's networking theories. I mean, why doesn't Microsoft produce all Windows applications itself? Why not make it a "closed" operating system? (ignoring the anti-trust concerns) It's more efficient for them to concentrate on certain areas (OS, business & web applications) and leave other needs for others to fill.