Nah, it's just to mark borders. All Mevans has to do is do his coding magic and grant it to us.
Biome borders are roughly fixed. And have all those structures generating in the same place is hard work. Same reason we’ve got so few fixed structures.
Most of all, its against the lore, if there would be walls on every border the fellowship coudnt have travelled anywhere.
The only thing, I would like to see as some kind of border wall is the great hedge seperating the Buckland from the Old forest.