You can also sell ores to other people outside of populated mines.
Though I agree grain mills and lumber mills have no use.
Actually they do have uses. Immersion for role players. They may not have any tangible benefit from game play perspective, but role players would see it as helping out the town they are in or a job if they played that way.
This is not to undermine the prospect of giving them an actual use other then role playing, though, as I fully support the idea. I admit, the first thing i did when i wanted to do some log cutting was equip an axe and started attacking a tree.