A farming flag would be helpful that way if you want to pay someone to harvest and replant your crops you don't have to give them the build flag. With the build flag griefing is possible and happens time to time.
I think somewhere one of the devs said sub residences would be implemented. I couldn't find the post but they already exist, only admins can create them and I think it's more complicated than one command. You could create a sub res for your farm if this ever becomes a thing.
I have also heard of plans to implement this for normal players, if I'm not mistaken it was on one of Aikar's streams that he mentioned it. Aside from that, a "farm" flag would actually be a neat idea, especially since I'm guessing that the number of sub-zones is going to be limited.
Oh yes aikars stream is where I heard it. That's why I couldn't find a post I'm thinking it will depend on supporter rank like normal reses.
I think it's still a good idea. Maybe he wants people to access his crops, or it's a public farm and he doesn't want to waste the chest space, or ect. ect. Making options is making a good game better.