Saying that you can experiment in Pre-Open Beta is false to be honest.
In the end, things aren't right... not at all!
If someone used to "experiment" in pre-open beta when no diminishing existed and then had to quit the game and wait for OPEN BETA, they will choose the wrong class!
People left the game when Dodon Barrage did 3000 Bleed on UD3 Mobs wont know what happened when they try DoTs in Open Beta either.
Pre Open Beta, as for casuals or pros, is for trying different basic styles. Maybe you like to heal? Maybe you see Tanking is the best way to go? Or maybe you want a Sword on your back?
Those are the questions you want to answer in Pre-Open Beta as a new (or sometimes even old) player.