Unless you're the type to upgrade every time a new phone comes out, I'm skeptical that a few scratches make that big of a difference by the time one plans on selling.
No like after two years, you can sell it to people or companies. Amazon for example, will give me $175 for my "perfect" condition iphone 4 which means no scratches, dents, and everything working. With "good" condition which means the phone still works but there are cosmetic problems, it falls to like $116 I believe. A huge drop for a couple of scratches that could be avoided with a simple case. When selling to individual sellers, perhaps the price won't drop as much but I'll still get more.
Plus, if you don't use a case, you'll love the look of the phone until it drops and it dents/cracks. Then you'll never appreciate the looks as much as if it didn't drop. If you had a case, you can always just take the case off and still appreciate the full beauty.