All my iPhones have been totally caseless.
1. A part of what you pay for the phone contributes to design and form factor; I like that aspect of these phones, so I choose not to cover it up.
2. If you're careful with your products (and I know you guys are), the software of your phone will make it obsolete faster (with upgrades that come with new handset releases) than regular wear and tear to the hardware.
3. A lot of the protection you can buy for your handsets actually do significantly more damage to the hardware than no protection at all, ironically (as evident by the issues brought up in this thread, as well as issues such as the glass back of iPhone 4 being broken due to dust build up behind hard cases).
The last point I think is most relative to this thread, but all three are why I choose to go caseless. Does anyone else do the same?