If the point of this Closed Beta was to actually playtest it in order to catch as many bugs and issues as possible, especially the more serious ones, then the wiser move would've been to conduct an Open Beta so anyone actually interested in extensively playtesting all of the game's aspects could do so, while those only interested in checking what's new could do so as well.
Well said, I've contacted Larian's support directly through the feedback form and expressed my will to playtest the thing, yet I was just redirected to the Steam sign-up page, sadly. Intentions behind this playtest are mysterious indeed, almost seems like they knew it needs a ton of work and were hesitant to put it out in the open and receive proper (and sometimes improper) feedback...