Alright, just to clear this out, I am sorry you feel this was a waste of time, It wasn't supposed to make you feel that way, it was maybe too story focused (behind the scenes, so to speak)
You all forget a key part of this all that i explained to Aroona after the event.
Your entire intel on the cultist is based on what the disguised cultist said, Joe didn't mention how many they are.
As Joe has pointed out at the end, he found the corpse of the real agent which was dead already a day before you arrived.
Also, Joe did try and offer his help at the end with finding out the real intel by reanimating the head which was declined by Lilandris (rightly so, but still).
Maybe I should have made it more obvious.
It was supposed to make you feel a desire to discover who those people are who pulled you that far.
One more point I'd like to point out, The real cultist, probably never entered Ogrimmar.