Kill Ten Rats is wondering about ghost towns in WoW and Asheron's Call. To me, there are two factors that decide whether a city becomes a ghost town or not. And geography is not one of them.
Geography only matters when your only means of transportation are your legs or your mount. WoW has many other forms of transport. You can fly, ride a zeppelin or a boat or teleport. The reason why Silvermoon, Darnassus and Exodar are empty is that they're dead-ends. The contrast becomes obvious when you look at Orgrimmar or Ironforge. Orgrimmar has numerous flight paths to almost every Horde outpost in Kalimdor, and has two zeppelin routes to Eastern Kingdoms. Ratchet is just a short flight away, and provides yet another path to Eastern Kingdoms. Conversely, Ironforge is only a short flight away from Menethil and has ample access to most Alliance outposts in Eastern Kingdoms. In other words, Orgrimmar and Ironforge are travel hubs, while Silvermoon, Darnassus and Exodar are not. The contrast becomes even more obvious when you compare these cities to the current de-facto capital, Shattrath. Not only does this cross-faction city provide flight path access to all Outland zones (the only zones that matter to the majority of players), it also provides portals to every other capital. It is a travel hub on steroids. Even if you didn't have anything to do in Shattrath itself, you most likely travel through it.
So, if you wouldn't want to go to ghost towns to travel past them, why would you go there? Auction houses? Every capital except Shattrath has one. Class trainers? Nope. Stormwind has a shaman trainer and Undercity has a paladin trainer. Profession trainers? At pre-60, that is the only valid reason for a jewelcrafter to go to Silvermoon or Exodar, but that doesn't help Darnassus. After level 60 you can do your business in Thrallmar, Honor Hold and/or Shattrath. That leaves other players. But nobody goes to Silvermoon, Darnassus or Exodar because.. nobody goes there.
Unless the designers can provide a compelling reason to go there, the three cities will remain ghost towns. One such compelling reason would be new boat paths. The two boat paths to Theramore and Menethil at Auberdine should be moved to Teldrassil or Azuremyst, and there are two unused harbors at the outskirts of Silvermoon. If these harbors would be the only link to Northrend, the population of these ghost towns would change overnight. Flight paths should also be enabled, although this would require "de-instancing" the Draenei and Blood Elf zones.