Does the game mention that an FC house is meant to be used? If something states that it is meant to be used, does it also mention what being used means? I have alts across many servers and I have many personal FC's I use as a bank and a few of them have homes for the simple reason if I ever feel like expanding I have an FC house in waiting. If that standard fits for personal plots, why is the sake standard not allowed for an FC which is simply a collection of individual players?

FC standards only harm players that feel the need to push their own standards on others. To you an empty plot may not be in use, but to the owner of that FC wanted a place they could call their own just for the sake of calling it their own. May not meet your standard of use, but if it serves their intended purpose for buying the plot that to me seems like a case of it being used.