If all your concubines have good genetic trails like Genius, Strong or Attractive, you have a better chance of having at least one kid with one of those traits. I think you might also get some extra prestige for having more concubines, but I'm not 100% sure.
Other than that, I can't think of any other benefits either. Keep in mind that you'll still have the ability to take concubines even after you reform the religion and gain access to other succession laws, though. --- Currently playing: Trails in the Sky FC (PC)