Is there proof that the positive effects are still there after you're hooked? Or are the "positive effects" at that point just a cessation of the negative effects of withdrawal?
Now, the euphoric effects that you get at first, those very rapidly go away with tolerance. With habitual use, you probably only experience a tiny shadow of that with the first hit of the day, or a respectable replay if for whatever reason you go a couple days without (which is heightened by the cessation of withdrawal). The nausea and disorientation also go away, which is nice since otherwise it would be a problem.