This is basically the first step in the creation of a unified, single realm across whole regions of World of Warcraft. Similar in concept to a PvP Battlegroup, this basically allows you to treat players from other realms in the shared virtual realm as if they were on your selfsame realm, including all the benefits of guilding, grouping and raiding.; This is an enormous game changer, and I admit, I don't think I ever expected to see it happen. For guilds on low pop servers, this drastically increases the potential player base that you can recruit, for just one potential benefit.
Do remember, however, that this is the PTR. As Ghostcrawler says:
Take data-mining and even PTR itself with a grain of salt. PTR is our Petri dish and not every experiment works.- Greg Street (@Ghostcrawler) June 12, 2013
Stay tuned for more news -- this is definitely an exciting change.