I noticed something funny while attempting to weed the garden this morning: the most difficult part to weed is right next to (or inside!) the walls. The middle of the garden I can just mow. Two swipes with a fancy cutting machine and the grass and weeds are all under control. In between the carrots and tomatoes I can easily trim, hoe, stomp down the weeds, or just spread ground-cover cloth and top it with mulch.
But getting right up to the wall is tricky. Often the weeds are growing flush against it, if not into it. The mower can’t get in there. You have to pull each weed away from the wall–sometimes through the wall–before you can cut it. Vines love the wall and slowly beak down the physical structure of its surface.
Like the Irish, I have piled up my plowed up rocks into a wall; grass grows between the stones and by the end of the season, the wall has half-disappeared. At that point, just pulling up the weeds disturbs the wall, because their roots are growing under the wall. It’s often faster to just move the wall and then put it back, crushing the weeds. (Or, of course, not letting them grow there to start with.
Without maintenance, the rocks would shelter the very plants that split them apart and pull them down.