I have a feeling that the game is probably pretty much finished and that's why they're waiting a month to start the next phase of beta. They'll just be using these next few weeks to make tweaks and fix bugs, so that when the beta does begin players can focus more on playing the game instead of all the bugs. I'm hoping they took a note from SWTORs recent release and will try to fix all the known bugs before the public gets access to the game.
Then the public closed betas will just be to test their servers. The beta events will also be on the weekend, so it gives them all week in between each event to make more tweaks and fix more bugs. Each weekend you'll have a chance to beta test the game, and getting in for one weekend doesn't mean you'll be playing in any of the subsequent beta weekends.
They've said that they'll need tens of thousands of players to test the game, and they've also said that most players will get a chance to test it.
There probably won't be anything like an open beta, but they've said that there will be another chance for people to apply to the beta. I'm gonna guess this will happen shortly before the game is released so that they can test the servers on an even larger scale.
I'd say if we're lucky and things go smoothly the release date will be in May, but no later than June.
Disclaimer - most of what I said is just speculation, and some of it is having high hopes, and some of it might be giving them too much credit, but I'd like to think they're right on track to delivering a quality product.