I like the concept, but it is too slow.
I've always been intrigued about implementing Diplomacy in MMO's. But the thing that sucks about parley in the main storyline is no matter if you attack or use parley you get the same end result!
Sure the NPC conversations and animations are different, but you convince the beastmen to consider joining your cause against the Empire.
I remember Vanguard they had the annoying gibberish with it tied to a card game.
The initial premise, I thought was to haggle with the NPC's selling and buying gear from you. As well as some story elements.
However either way whatever minigame they add. It needs to not take so long, the effort in doing it taking some thought, and the player being rewarded for taking the time to do it if successful.
Such a mechanic will be abused by those that are good at it. So... They can put a limit on how often you can parley in a day. (unless it's a story/sidequest). That way they can explain away that the NPC has grown weary of your conversation and needs to take a rest or drink a glass of water. lol