I don’t know whether they picked that material for sentimental reasons (all iPods back to the first one – except the smaller ones – had that back) and whether you are just supposed to accept that back that’s slowly getting more and more scratches. (I won’t use cases. Never. I have to be able to put your gadget in my empty jeans pocket. If I can’t there’s something wrong with your product.)
Apple knows and used better materials. My trusty six year old brushed aluminum iPod mini pretty much looks like new and has been treated far worse than my iPod touch.