Take a look at the title bar of Project Spartan in the screenshots above. You may notice that window controls and title bar are on the top and below which tab bar is displayed. As you can notice, it is a waste of pixles. Project Spartan team is aware of it and they have now included a experimental feature that will tweak the title bar UI. With this feature on, you will have one integrated bar on the top which will have both window controls and tabs. Take a look it below.
When Microsoft demoed Project Spartan in January, it had UI similar to the above screenshot. Hopefully, this design will be in the final version of Project Spartan.