There is a difference between client side phasing and multi dimensional phasing, what people want is multi dimensional, what we have seen so far, is client side.
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How can it be client side ? if the NPC doesn't exist on the server how can you interact with it ?
I think you have it back to front, what WoW does is more client side as it changes the zone which is only a graphical thing, it also has server side phasing with NPC's though and ARR already does this.
Technically... Every zone in the game is an instance, or you wouldn't be loading into them when you zone... There is no such thing as open world content, it's an illusion.