Basically, the farm will have an 'interactive use' in that you can redistribute the population within the city. For example, say you have 2000 FS and a lumber mill that produces 3000 per hour and in total requires 500 farm space. The other 1500 FS is take up by troops ready for an attack but you'd like a bit more. So, you can go into the farm and reduce the number of people working at the lumber mill to say 250, so your production is reduced to 1500 per hour. If you reduced it to 0 it would obviously produce 0.
You would be able to distribute FS between all buildings and free space in a similar way so, for example, once you have researched everything you can in the forge you can put it out of commission in case you need it again.
I hope this all make sense but if there's anything confusing let me know
Also, as a side note and an extra idea, maybe any free farm space you do have should count as a weak defensive unit, maybe 10 points against each attack type per FS.. Not sure how this would work though.