Suppose I got a U2715H and a 34UC79G, and suppose I am writing an email while looking at a source page; I drag out the source page from the Maxthon window which is on my U2715H, and toss it onto the 34UC79G. I finish the email, hit the send button, and now what? I'd still like to keep the source page open. I'd hate for it to be left alone in the dark, across the great schism between screens. I can drag it back to U2715H, but it'd still be out of its original circle.
All of this, of course, is hypothetical, but highly probable and realistic.
That said, I agree with you it's better than nothing. I didn't start this thread for my benefit. I am actually a one-window, multi-browser guy.