"But we still don't know what the Jailer wants!!"
To be honest I don't think that's a good enough reason for Activision Blizzard. Something tells me that Bobby Kotick doesn't care too much about the Jailer's motivations.
... And maybe the Jailer simply isn't meant to be more than a generic evil villain. People here constantly theorize that "Oh maybe the Jailer is the good guy, maybe he wants to free everyone, maybe he's secretly the good guy!", but Warcraft is such a simple story. If a character constantly spews nonsense like "Your world is mine!", "You cannot stop me!", "No one can escape me!", "Foolish mortals!", "I will destroy you foolish mortals!", then maybe they are indeed just evil