If you saw this week's Ask CDev answers, you saw this particular exchange.
What this ignores is that, basically, warriors have a union. It's called war. It's baked right into the name. The whole purpose of warriors is to get out there and fight people - we're not about getting together to form a collective. It would never work. Armies? Sure, we'll be in those, but the other army we're about to fight usually has a bunch of warriors in it too, and it would be weird for us if we knew all those guys from our weekly meeting or whatever. The fact that the Cenarion Circle has druids from the Horde and Alliance in it and they kill each other seems weird to us. We don't kill our friends. We kill our enemies.
So no, there will never be a warrior union. If there was, we'd refuse to join it. We're busy killing on another -- it's what we do.