I don't think the restrictions reduce the amount of spam attempted, but it does localize it for easy removal. Since there is no way we could read every post made in every forum having it kept to one area makes it easier to manage. Spammer are notorious for being lazy, they will post their crap where ever they are allowed to. If we only allow them one spot, then we can see it quicker.
Sort of like a "dog run"? aka: the spot in the back yard that's designated for dogs to crap in?