With both UIs, Microsoft intends to achieve a balancing act, as both of user interfaces together harmoniously is an important part of Windows 8.
You can seamlessly switch between Metro style apps and the improved Windows desktop. Existing apps, devices, and tools all remain and are improved in Windows 8.
As posted in the official Windows 8 blog;
the role of the Windows desktop is clear. It powers the hundreds of thousands of existing apps that people rely on today, a vast array of business software, and provides a level of precision and control that is essential for certain tasks. The things that people do today on PCs don’t suddenly go away just because there are new Metro style apps. The mechanisms that people rely on today (mice, physical keyboards, trackpads) don’t suddenly become less useful or “bad” just because touch is also provided as a first-class option. These tools are quite often the most ergonomic, fast, and powerful ways of getting many things done.
Windows 8 is reimagined from the chips all the way to the user experience, to enable new scenarios, new apps, and new ways of using a PC.