I use a screen protector on my 5d MKIII and it works great for me. It was much less expensive than a possible replacement of the glass. I think it was around $10 from xoskins. It also covers the top LCD screen. I am not worried if I need to replace it because they cover it for the life of my camera.
+1 on XOskin: http://xoskins.com/canon-5d-mark-iii-screen-protector