The idea that the customer owns and builds the brands feels a little unrealistic. Certainly customers feel a great deal of "ownership" and loyalty to brands that they feel help to define their lifestyles but that brand had an identity before they discovered and adopted it.
This applies more to brands that have gone through an identity renaissance like Hush Puppies did when they were brought out of a 20 year slumber by people looking for something that was outside of the mainstream. The revamped image due to this new customer is a better example of customers driving brand identity.
So where does that leave the agency? Brand builders and stewards open to partnership with consumers? Ringleader or barker? The smart agencies won't engage in a power struggle with the passionate public but harnessing the power of consumer generated content has yet do be done in a way that makes it feel like a new model.