> By analogy, there's a reason that many ancient cities were circled by a wall.
Walls around cities were likely very poor at stopping small, stealthy groups of infiltrators. They were designed for much more brute force attacks. Apple's walled garden helps quite a bit with the deluge of crap that would be available without it. Without it there would be an order of magnitude more crap (in quantity and quality). That said, there's a vibrant black market for people that can't stand the oppressive policies of Apple.
Additionally, once you have a wall in place, it's easy to make a decision to tax certain types of traffic through it because the capability is now there, whether or not it's in the best economic interest of the people inside. Apple didn't skimp on this area. The wall was erected with tithes and taxation in mind, and collection booths at all the gates.
So, does it help? Well, it prevents roving bands of bandits from riding in, terrorizing and robbing the people unlucky enough to be in their path, and making a hasty exit, so yes, but if you're a tasty enough target, getting past the wall isn't really a problem. There are myriad ways to do that as long as you're careful. For example, the numerous secret tunnels through the wall. They aren't large, and they are constantly being filled in by the city engineers, but there's always some they haven't found if you are willing to ask the right people (or dig your own).
Okay, I believe I've tortured this analogy enough...