An example showing why single window interface is bad.

Microsoft strangely pushed their Ribbon interface, and with that they also pushed single window interface. You can say, "Oh, there are tabs. So, you can open multiple documents at the same time." Yes. It is true. But you just said that you were not a power user or even a good normal user.

Tab serves its own purpose. But being able to open multiple window at the same time is very important. For example, you can compare contents of two documents ( either they are from two different application programs or one application program. )
But with tab, you can't do that. There are certain implementation of tabs where you can split views and each views can hold their own tabs. But MS's implementation is not like that.

 

One table you want to reference

 

You want to create another table based on the table 1 show above

Actually, they are in one document. The document has multiple tables and multiple forms. You can run into situation to make other tables or forms as an reference for a new tables or forms. However, MS's single window approach doesn't allow that. If you choose one tab, it covers the whole screen or the whole window.

This is very silly and unproductive UX design. Although the designer or the lead of the Ribbon interface and thus all new direction for MS UI has said that it was great, but I found out that it was more counter-productive mostly. Many people complaint it and even one defense by MS people was posted to MSDN home page. ( I don't remember it. )

Now, Apple is trying to push their own single windows UI. Good thing is it looks users can choose by maximizing a main window full screen, but still, if it is not  a kiosk app, I don't think it is as productive as traditional multi-window UI.

Probably new users can find the single window UI feels easier to use and prettier to look, but if once they get used to it, they will not feel that way anymore. ( One way to prevent it is not to show multi-window approch to them. )