If it's that big a deal for you just get a case... But get something tough, like the Otterbox. On the other hand, if you're the kind of person who's going to replace the phone in 6 months then why worry?
I personally have never bothered with cases. When I'm mountain biking, for example, I stash my phone in my backpack where I don't expect it to get hit if I go down. Otherwise, if it's inconvenient I just don't take the phone with me, but then I don't think it will kill me to be out of touch for an hour or two.
It took me 2 years to finally get a scratch on the front of my display and it isn't really all that noticeable. Without a case, the sides and back are going to get marked up no matter what you do. If you're keeping the phone for the long haul it becomes a fact of life. And I think the plastic back on the Lumia should age pretty well, certainly better than the aluminum back on the iPhone 5.