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?
What you are talking about would require SE to make NM's that are nearly unbeatable. What about the other NM's currently in place? The normal monsters? What happens at PS3 release when every new player crowds around Rank 1 camps fighting for mobs?

I'm not against massive PvE events like the ones in Guild Wars, I'd love to see that.

But changing the entire claiming system just to implement a couple megabosses (your solution)

OR

changing the claiming system so we don't lose exp on unclaimed monsters (their solution) seems like opening Pandora's Box just so you can kill that coworker you hate. It's a huge can of worms and - I agree with you - ultimately a waste.