what is the point of a system where multiple parties can participate if only 1 team gets the rewards? There is no point, that is the problem. And yes if they made world NMs that were actually difficult enough or scaled in some fashion to the amount of people fighting it, then it would serve a purpose, in creating new and interesting content that many people could participate in, in the open world.
Go look at guild wars dynamic event fights, everyone who participates to a certain level can get rewards, it takes place in the open world, and the reason people work together is because they need to, in order to win. Its not just a zerg fest, because they dont design the fights to be that way, it has multiple levels and multiple strategies.
Im not saying they have to go this way, but there is no advantage to this system of having multiple parties participate, if there is no bonus for participating, in fact, the system as they describe it, the only advantage you get is if the other party dies, so what type of world do you think thats going to create?