I see the sets as a way of defining constraint, i.e. establishing what an approved construction standard is. It then becomes possible for someone with extra bricks to invent a new model that fits in with the same scale/standards. Look at the fan-created models that scale to the modular building standard established by the Cafe Corner/Greengrocer sets for example. Sometimes a set of reasonable constraints leads to more creativity than there would be in a completely open environment.