OK, so it scratches. I'm sure a large percentage of iPhone users have normal wear and tear on their phones. I, for one, am a user who typically does not like to use cases. I am, however, trading in my iPhone 4 of two years of use through Gazelle and getting $163 back for it with regular use.
Of the options they give (3), they are "Broken", "Good", and "Flawless". The price difference for my return would be about $5 from "Good" to "Flawless". I guess my point is, that if it scratches then so what, it's normal wear and tear. If you want to keep it pristine for years to come and upgrade to another phone down the road- whether you keep the iPhone 5 or trade it in, the money-back price does not change much. Don't get me wrong, there are other reasons to obviously use a case, but in my example, keeping it in pristine condition does not effect your return if you end up trading it in.
As another commenter said- iPhone in the right pocket, keys and everything else in the left.