It is not a reflective shield, there is no harm done DPSing during it. It is Reactive, reacting to the number of players in the area. Stopping DPS during the shield just prolongs the fight.
Other than that, I basically agree with you. I am not a huge supporter of them yet, I just do not understand this negativity directed towards what is an enhancement of the old Expert Rift system and provides opportunity to expand and varying levels of challenge. It also affect Raid Rifts which still exist in no way.