Except some Batman movies have made a point of unmasking him at key moments, particularly in Batman Returns, where it was done very awkwardly and gratuitously in the climax. Even the '66 movie was written to give Bruce Wayne a larger amount of screen time than he usually had in the series. But the point is, we did see the lead actor out of the mask. It wasn't all mask all the time. But David Prowse wasn't a big-name actor. If they'd cast, say, Paul Newman in the role, you can bet they would've unmasked Vader halfway through the first film or ditched the idea of a face mask altogether. A famous face is a valuable commodity. It's worth money. If a studio spends a lot of money on a famous actor, they generally expect that actor to be seen. This is why Sylvester Stallone took off his helmet as Dredd. This is why Tobey Maguire was constantly unmasked as Spider-Man. It's about the studio wanting to get the most for their dollar. In this case, it probably helps that, though Urban's star is on the rise, he's not a superstar like Stallone, not quite a household face, as it were. He's more a character actor than a leading man whose face on a poster automatically sells tickets or DVDs. So there's not as much pressure to make his face visible. He was visorless at least twice in the films: in the climax of the first film, when Magneto took his visor to force him to keep his eyes closed, and in The Last Stand when Phoenix was holding back his eyebeams.