I see where you're coming from, but I think it's more for the fact that you have no control over which plants will get fertilized by a stranger who has no stakes in your garden. Someone could just come along and fertilize one, or half the patch. From previous posts, it sounds like they want to keep adjacent seeds in place as long as possible for cross-breeding, and cutting down the duration would ruin their system. (I'm not so knowledgable, so I hope I'm not messing up what they're saying.)

I can see your idea working if it was just left to tending someone else's garden. I know what you mean about doing something nice for your neighbors, though. I always wanted a drop box that as a neighbor I could leave food, or some kind of gift, without having to be friends with them first in order to send a message. I noticed that placards have a box on them, and I just wish I could place something in there.