But it would work - people have the option of joining a guild because of its benefits. They can garrison troops in the Guild city if they so desire, they will want to because - if the Guild city is plundered, they lose X amount of resources, and there is a temporary loss of Guild bonuses.
That is if the city can only be plundered, and not actually captured.
I think Plundered is better. Otherwise there will end up being a few massive guilds that wipe out all of the smaller guilds before they have a chance to get going - aka we want the MMORTs to be viable for future players in the years to come.