I tend to agree with Zets that the BIG Beta will be for testing things that are much more broader based like the unlock and achievement systems and how the master servers will react to a bigger player influx.
Of course if there are any major bugs that surface because of having lots of players testing the game then it can only help the final product be an even better one and a big beta will certainly help TWI focus the game on the things players are liking whilst helping them either polish or remove those that are simply not working.
It also helps with what may need to be patched a later date as big player counts usually throw up some weird stuff as well in a beta test...