I have read where people have shattered their glass screen protectors. I don't even want to waste my time or money on those when the films work perfectly and resist oil. If my protector gets a scratch which has happened, I just peel it off and out a new one on.
I wondered about that actually, I recently saw someone with a cracked screen iPhone 6, but he seemed to have glass screen protector on it, and the phone was inside a case too, I couldn't tell if the screen protector was also damaged, so they don't do much in term protection, just for a better clarity?