The beauty of 'open' world content is that you go to an NM or dungeon or some 'activity' by yourself or with a group of friends, you may run into other people on the way or inside.

Open world content would not restrict the activity to only those in the party -- you could have people outside the party participate, help or just watch and admire. You could even have situations where you have to compete with another group of people for the honour of fighting a mighty foe and claiming its defeat and riches.

It creates a more 'realistic' 'livable' atmosphere of an immersive world where you run into fellow adventurers who are trying to do battle against the same enemies you are facing.

That being said... in this 'modern' world where we are faced with more people who are anti-social, do not get along well with others, or are into 'grief' play, this faces problems. By ensuring we are all in an instance, it removes the 'having to interact with others' that some people do not like.

Take for example, one of the very few activities that are 'open world' right now: Caravan Escort. Those who have participated enough will have seen people who complain that they can not get in, or someone else got in instead of their friend, or that someone is running to the end to get seals without working. This 'open world' content allows for us to interact with people we would not have otherwise planned to. And this can end up in situations that may not have been desired.

By not having open world content, in essense saying: If you all can't get along with each other and play in the same sandbox, you will go play in your own sand boxes. And this is where we are going.

It has its goods, it has its bads... And until we can solve the underlying issue in some people who do not understand how to interact with others in an amiable fashion, this may be our best compromise.