The first thing I asked about when the rep at Costco handed me the phone was if there was a case in the accessory package that came with the deal. The phone felt too thin and too slippery naked. He put the silicone case on the phone and it has stayed on ever since. Does not add much bulk. No screen protector. I do have a leather case that the phone slides in to when in my pocket. Phone is still pristine. (It is hard to find nice, real leather cases without a belt clip).
Hard for me to believe that the HTC One, or any phone, could go two years unprotected and unscratched.