No mobile glass is 100% scratch-free. Gorilla Glass 3 can still get scratches from small particles like sand, but with proper care your phone is unlikely to get scratches. I would say get screen protector if you want to minimize the chance of scratching as much as possible.
In that particular case I would rather recommend a good screen cover or an ideal case to protect your device. Sometimes the hardest thing to withstand is a scratch that inevitably leaves a deep mark in your screen that is sometimes too annoying and you can do nothing about it.
Hence, there may be few reasons not to use any case for your device but there are several reasons to use one. I was a victim of these unwanted scratches few months ago and that made me realize the significance of having a screen protector for my phone.
As far as i am concerned about your Moto X, there are a number of screen protectors that you would like to have a look at. Honestly, when I was searching for an ideal screen protector for my Moto X, I found a number of good stuffs in the eBay, the store like gearmaxx and other have a lot to offer.
The one I have been using now is a product which gives a better protection from heat and leaves no impression of your finger prints, so all in all it is the most relevant one I could find in the stores. You can choose one for yourself from the gearmaxx or other brands from eBay.