I'm not speaking for myself. Personally I rather hang around in the player-wise empty capitals, because they're empty. And I'd love to see a Goblin and Worgen capital, but that's obviously not enough to make it happen. I'm not saying Silvermoon is bad designed, but there's almost no one around, so from a technical point of view it was a waste of time to create such a big city with unique buildings, when no one even goes there.
Have you been to Undercity? You can remain mounted there too since... years now. Anyway, I don't think it's necessarily only the force of habit. Those cities are central and most newbies and toons sooner or later end up there, but most importantly the overwhelming majority of people likes to socialize. Since theres no Dalaran and Shattrath around this time, maybe we get to see some activity in the other capitals.