You forget the former Robelia was up to the task while the concubine isn't. Also the actual Robelia might just betray him to force the divorce, if not to eliminate him should she free herself at the price of the emperor death. Can't be ruled out that the emperor has a kink for wiskedness in women, rather than seeking "understanding" -- which (according to Alexendro's words/thoughts himself) I remember annoys him in the concubine at the end of the day.