I doubt most people would zerg open world NMs if the loot is only so so. I'm sure most people would go to an instance for the fun and challenge, and I assume most instances would allow only 1 or very few groups in. Allowing a passerby to help out on an "open" world NM can allow for a more interactive co-op world. Open world NM become mini public quests if you will. Instance NMs would be more for party play.

1 claim system can be boring because you don't interact with anyone else. Its just you and your party/LS for everything. Having a little spontaneity can't hurt.

I don't think anyone here wants to have a repeat of a hundred people spawn camping an open world NM like in XI. Tho its less likely now that open world NMs spawn much faster and in more places.