Medium to large cities already have lots of building slots. Smaller ciities, townslands, and others may start running short. Which, really, is the point of having limited slots. If every region had enough slots to build everything, then what would be the point of having slots?
Having a building that by not having your basically writing "For sale" on the side of your city reduces the number of slots, by one in all cities, in the entire game, for the rest of time.
I really can't see any way to justify not having a lighthouse at every region that can make one.
This sea feature seems so exciting, I am looking forward to it 
I just realised whom would have made awesome use of this feature. Damned Aurvandil burning them to the ground...
