Insane combos. Garrison post and archers watchtower in the same zone. Warlord keeps spawning goblin slingers behind walls, and they get off several attacks before you can destroy the wall, unless you climb over it. Even then, he has at least two more zones just like it and they are easily replaceable for a relatively low cost. Better if those goblins have to move twice to reach a tower, since they already have other ways to protect them on the way besides walls, like fortified position, standard bearer, guards and especially armory.
And worse, with such a rule change he can get a lot more total outposts into the arena, which will make his barracks channeling ridiculous fairly quickly.
That doesn't sound fun. It sounds unfair. I think if I played against a warlord and this was a house rule, I would not want to play.