Most of us have bought the N9 partly because of its looks: it is a real jewel. So, then why cover it up? You can then just as well place it in a glass showcase in your living room.
The phone has to be used, its smoothness and sexy slipperyness has to be felt with your fingers, it has to be shown off to the casual onlooker. Sure: you get a few scratches here & there (especially the silver part around the camera lens is sensitive), but that is OK: that is how it becomes really YOUR phone, and not just an anonymous piece of electronics - just like your leather jacket becomes more comfortable over time, despite its stains & scratches.
So: don't use a case - keep all its naked beauty exposed!