In my opinion, it is a mistake to not use Hive as the primary currency for Hive based game. And the Hives gained playing the game should not be easily returned to the player Hive account. This would prevent people that do not care about the game, but want only make money from the game. If the primary currency is not Hive and can be sold on the Hive Engine, its value will go down rapidly.
It is OK for the premium currencies to be Hive Engine based.