In my view, the goal of the current EA build is not to test for balance, but to test systems.
Balance can be adjusted at the very end of the game's development. (The playtime-to-level progression could basically be adjusted in an afternoon of light work. Obviously this is merely one of many, many aspects of balancing the game.)
But some systems can't be corrected in a fingers' snap. If Larian's implementation of Extra Attack, which the full martial classes get at Level 5, is highly problematic, they should want to know asap, so they have time to rework it before the game is released. (Possible problems : inability to swap an Attack for a a Shove, ability to use an extra Action instead of just an extra Attack, inability to move between two attacks ... )
Likewise, Level 5 gives us access to Level-3 spells, and again, there is a lot potential for things to go wrong there.
So, there are good reasons to want to test Level 5 now. Balance is not the top one. (Although it is one, since it would allow Larian to see how powerful Level 5 is.)
