Last time I checked the cross realm dungeon finder only works with realms in your battlegroup. Battlegroups are limited to one data center so they don't have to worry too much about how the data gets from data center to data center and if they need to do any encryption or special handling of the data.
This is extending it to all players not matter what battlegroup they are in. Spanning data centers. May seem like a minor point to you but even so this does add another layer of complexity.