The "ruse," the main story arc of the season, is unbelievable and not entertaining. Bunny's 3rd season (my favorite) experiment may have been improbable, but the fantasy illustrated, in a plausible and even handed way, a potential strategy for dealing with a social problem. The fantasy contemplated a potential solution to a real problem.
Mostly, the ruse shows how wacky McNulty and Lester are. The scene where McNulty hears the FBI profile was funny,* but the rest of the comedy has fallen flat for me. And I don't know what's going on with Lester, who has suddenly become a wacky schemer who sometimes has weirdly melodramatic interactions with Clay Davis.
At best, the ruse illustrates how politics drive local government decision-making. But this point has been amply made by the previous seasons. Those previous seasons were more compelling because you felt that they showed how the world really works. Everybody has competing interests, and those personal agendas conflict with community goals and institutions. The results may be unsettling, but each step along the way was believable. And I don't believe the ruse.
*The humor is tempered by the appearance of an FBI director who only cares about his public profile. We know that he's only interested in his public profile because he walks into the room, tells you about his Larry King appearances, and then walks out. He does nothing to advance the plot, he only has a cameo appearance for color. It's broad and silly.