Making it an open beta will gain some popularity, but will sort of destroy the meaning of what a beta is all about.
Given that it's a closed beta, they'll get a small group of serious players to test the functionality of the game. That said, we'll see how the 360 version goes. They will most likely do the same when PS3 and PC beta comes by.
Open beta's are in fact great to have, you gain a few friends who are also playing, but you're also testing the game as well. I guess you could say that Crytek are serious about making a stable game to play.
A beta is a privilege. If you're able to access it, then that's good. But just remember this, if you really want to play the game, just wait till the demo comes by, at that time, most of the game will be playable and working, whereas we could expect some bugs/glitches within the beta's.