If the app is JavaScript heavy anyway, and it likely is if something like this is being considered, you have that all loaded into each tab/window instead of just one. Also you no longer have what might be small tightly coupled app elements contained in a simple tab where they can't get confusingly separated and where the app has to work harder if it needs some for control (a modal UI element being the most simple example).

Though I think you misunderstood what I was getting at with the tab/window thing. I was pointing out that the general "why X when you can Y" stated in the parent post isn't a great arguement, on its own, for not X.