I'm hopeful, it certainly sounds like it'd be worth playing. It does address a major problem with MMOs in general. I do worry about making the world too dynamic though, some degree of permanence will be wanted by people, just to feel at home in the game. Places mean something over time (Orgrimmar, Goldshire, Icecrown Citadel, etc.), too much change and people could find they were connected to game world that simply doesn't exist anymore. Though I imagine they'll be testing out things like this as they go through Beta versions and whatnot.
I think that's fair. I also think it's fair to put some impetus on the players to make them permanent.
If you've made your home in Stormwind, and it's actually possible for Stormwind to fall, it makes it all the more important for you to bother defending
Stormwind if it ends up under siege. And not in the "I guess this event is kinda okay" kind of way, but in the "Holy hell if we don't win this Stormwind will actually be GONE" kind of way.
I also supsect that the companion game is going to allow for the creation of assets that the developers can use to actually keep up with this kind of content. Say they issue a contest for the best "Ruins of Stormwind" and use the winner as the basic landscape they alter as they see fit for the actual EQN game, should Stormwind fall. Or, you know, something more reasonable like "Pick a building in Stormwind and show it post-Siege, should the city fall."
Seeing your work in the game, for however long it becomes an actual part of the environment, would be hugely gratifying for those kinds of designers, I'd imagine. Not to mention it could come with a unique character prize, some f2p currency, etc.
Basically, my point is that they're essentially in a position to crowdsource content in a way that actually enables the world to be as dynamic as they want for it to be, if they handle this properly. And my god do I want that.