When you click something in the main menu of the XP 6.4, it opens a new tab (target="_blank"), which is kind of annoying. The main landing page (http://url.to.enonic/admin/tool) only contains the main menu, which is always available in the top right anyhow.
If I for some reason want to open the content studio in a new tab anyhow, I could just cmd+click it, or right click and “open in a new tab”. But there’s no use case for it imho.
The home screen will get more features in upcoming releases, and the idea was that it should behave like all the other tools. The idea is that users will get confused when a new window will open or not if the behaviour is inconsistent.
Also, the reason for using tabs is that each tool can now be implemented using vendors desired frameworks etc, simplified deep linking. This is similar to how you see google doing things - using the browser to isolate and speed up things.