I would be pretty devestated if leveling in dungeons became the primary way of growing in experience, as that would mean that people would 'grow' seperately from those on their own server and would be less likely to get a sense of the community they are becoming a part of. That sounds like a very sad situation. I feel that a instances should be for special items, end-game content and perhaps special missions. But you should always have a good reason to party in the open world, too, and at the moment I believe experience is the main reason for people to do that. Well, in my limited experience of FFXIV, anyway!