If you asked me five years ago, I would much rather they merged the realms.
That was because five years ago, WoW was still actually an MMO, as in, MASSIVELY MULTIPLAYER. As you quested through the zones you'd meet hundreds of different players. There was no avoiding other people, and that was part of the fun.
These days, WoW is a singleplayer game. You play by yourself as you level, in your own private little phased bubble. Shared quest mobs means you never need to make groups, or even talk to the people you meet in the overworld, and when it's time for an instance or a raid, you just pop in the dungeon queue, wait a while, then zerg the entire instance with nothing more than a complimentary "hello" and "goodbye."
In it's quest for convenience, WoW lost the point of what makes an MMO. Getting ganked in STV. Grouping up with strangers to try and bring down some group quests in the Hellfire peninsula. Sitting in Org trying to organize a PuG for an Instance. The constant, massive War that took place at Blackrock mountain on raid night. These are my fondest memories of WoW. Not getting Shiny Sword of Shinies +2 from a boss with some faceless, mute strangers that might as well be bots.
So they stitch the realms together "seamlessly" in a last-ditch attempt to keep their playerbase together, but ironically, the entire concept of "playing together" has been all but ripped out of World of Warraft.