"clumsy user", "clueless user", "semi-rational user", "a sign that the user doesn't know what he is doing",
If you have ever conducted user studies of browsing behavior, you would see that the tasks that Aza describes are exactly the ones users perform in the real world. Why do you think it's unexpected for a user to pause a current browsing session and look for something unrelated, and wish to keep that search session separate from the previous one?
No, simply a new window would not be sufficient, because pretty soon, you end up with several different windows, and not all of us have the luxury of 30" displays to arrange them on. Yes, this is a window manager built into the browser, because default window managers have been inadequate in coping with the number of browser windows and applications users have open. If they were adequate, tabs would never have been needed in the first place.
From initial design sketches, this does seem like it will contribute its fair share to helping with the information overload problem. The only way to confirm that this is indeed a usable solution is to run it by users in real-world studies. You'd be surprised how much the average user or the power user differs from engineers and developers.