How hard is the glass on Retina MacBook Pro
As you can read on: http://www.anandtech.com/show/5998/macbook-pro-retina-display-analysis
the glass is not the same as on the usual MBP but instead created using two layers of glass.

But the thing is the new Retina MBP does not have the cover glass. That does mean that if you scratch it in a way you are scratching the real LCD and not just a cover glass which would be the case if you scrach the display on a MBP.

Hope you can decide from that whether or not you should use a screen protector.