Is there any reason to go beyond the Honored Status with the Black Prince? I already have the quest to get the 10 sigils of power and 10 sigils of wisdom. Unless having them exalted will be needed to progress the legendary quest line in the coming patches.
UPDATE 11/2/2012 (Patch 5.0):
Just got honored with the Black Prince and I can no longer get rep with him. So they must have it where you cannot get above that from killing Mogu or Ik'thik. Unless it can come from raids, but I have yet to do a non-LFR raid to try this.
Best Answer
For the time being there isn't a benefit to levelling your reputation beyond honoured with the Black Prince. Later on in the expansion with future patches you will need to get it to revered and then exalted, but methods will be provided in game through raiding and questing to allow for this.
In order to progress to the next stage of the legendary quest line (which isn't actually available in game at the moment) you will only need to complete the current quests, which as you've mentioned include getting 10 sigils of power and wisdom, along with honoured reputation.