Charlie, the game face is intended to help prevent potential catastrophic injury. While breaking an ankle sliding into a base is a terrible injury, when you consider the potential severity of a line drive to the face, it doesn't even compare. If you think about it, batters are required to wear helmets for protection. They aren't any closer to the pitcher than the 3rd and 1st baseman are to the hitter in many situations. On top of that, when the batter makes good contact the ball comes off the bat much faster than a pitcher can throw it. I know the defender has a glove and a better chance to defend themselves than a batter, but when you factor in the increase velocity of the ball off the bat, it probably isn't that much different. Why take the chance.
I do not think the game face should be mandatory but my dd plays third base and wears it. It doesn't make her a wimp or a girly girl and it doesn't give her a crutch. It does give her mother and me a little piece of mind. What's wrong with that?