I use my iPhone 4 naked and also did so with my 3Gs (which is now used as an iPod Touch). I bought slipcases for them when carried in my pocket (Sena cases - Kutu leather slip case) so they don't get scraped against any keys, coins or my pocket knife when carried, but to use them, they pop out of the slip case. My 3Gs is over two years old now, used daily until I got the iPhone 4 (my cell phone is my only phone), and looks just fine - no screen marks at all, and no scratches on the back.
I do go back and forth with using a screen cover (boxwave's antiglare ones) because they do seem to make it a bit easier to read the screen outdoors in sunlight, but they can also be annoying. So, my iPhones have probably spent at least half or more of their lifetime without one to no ill effect.
To be honest though, as long as the screen doesn't get marred, I really do not worry if the backs get a scratch or two. It's a phone, not a piece of pricelss art, and I also do not worry about re-sale value. I upgrade only when I'm eligible for a discount on the latest device, and my old one either continues to get used for something (like my iPhone 3Gs as an iPod Touch) or donated to a charity or re-use program.