Why Does Batman Show His Mouth?

Another reason he may keep his mouth exposed (despite being portrayed as having a complete poker face, able to hide his emotional state from anyone) is his occasional need to emote in a positive fashion as shown below.

Why does Batman not show his face?

In The Dark Knight Rises, Bruce Wayne explains to Officer Blake that the mask was to “protect the people closest to [him]” and that “the idea was to be a symbol—Batman could be anybody.” For Scarecrow, the Joker, and Bane, the mask is not a symbol of anonymity but represents their individuality.

Does Batman wear a mask?

Batman/Bruce Wayne's mask is unique in terms of how superheroes and villains use their masks, while other characters such as Spider-Man and Superman use the mask to reveal or conceal a hyper-expressive identity. Batman's mask is an amalgamation of his personas.