even though communites are realy good to have, even I am in one, it more or less is a chat chanel to some extent you use, becuse even if you can join dungeons and even heroic raids whit a community, you can not trade items whit them unless someone happens to be from same server, exept dungeon loot that dropps that is, and that is still only for boss drops.
I do think they could pretty much connect most realms whit eachother instead of just making server cluster connections now, that would not strain the server capacity at all, becuse technicaly connected realms are on their own servers still to some extent atleast, and kind of do work as one mega server.