Whilst I completely agree, Apple devices are designed so beautifully, and it's criminal to 'hide' their sleek lines away, if you use one of the better thought out 3rd party smart cases like those from Invision, that put a close fitting shield on the rear and leave the front glass completely exposed whilst in use, your not really losing the beauty of the device!
Every now and again it's nice to just hold the device 'naked', but since I don't spend much time admiring the metal rear, I don't feel I'm missing out having it protected properly. I also feel its vulnerability when it's not in some sort of protective case, and with a £700 investment I think it's worth just looking after it for when I'd like to upgrade to the next version.
Having dropped my iPhone 3GS and wrecked the LCD (despite it being the most ergonomic version of them all with its contoured back) and subsequently dropping the less ergonomic 4 a couple of times, I pretty much always have my 4S in at least a bumper with rear protection type case, and so far it's protected the new phone from a few dings!
The simpe truth is that Apple portable devices are designed to look gorgeous, shiny and slick, but they are not really designed to withstand much more than careful admiration! - a great example of this would be the mirrored chrome rear on some iPods, and the iPod touch - you only have to put it down once, and the back is marked!! - shiny yes, but only when you first open it, then it's shiny with imperfections :]