I think the reason why we haven't seen them, and likely not to, is because they don't want people to not run certain dungeons. If you stock up everything from that dungeon, you aren't going to go back to grind for the other pieces.
It's not an argument, it's fact. People will stop running an EX Primal once they have all weapons/better weapons and the bird. They might help a person with bonus, but running it again will reward them nothing of value... and people who play MMOs love having items that have value so they'll run the content for the valuable piece.
