Because there are lots of servers and new ones are introduced every week, it could be beneficial to have cross server GNW. I see most people who have different accounts in my server and those in the forums talk about how there are only one or two active groups in the servers. Slowly one of the groups in my server are becoming inactive and will only leave one main group. There is no point in having a GNW if they aren't "Great." The Sage Battles got better with the cross server addition and I believe that the same result would happen with the GNW.