There are things that work in some mediums that don't quite work in others. In the comics and animated versions of Batman, the mask is form fitting right up to his eyelids. When he shows any kind of emotion like anger (squints), surprise (wide eyes), or even gets knocked out (closed eyes) etc the mask and the eye whites follow. For live action versions, a mask that goes right up to his eye lids with lenses on top of his actual eyes are uncomfortable from the actor's standpoint and impractical from an effects standpoint. Like void said, in the absence of a comic-book world mask that fits your every expression, visible irises help a lot with expression for actors with much of their face covered.
In the comics sometimes there is an explicit mention of the lenses in his cowl, but whenever he takes off his mask, there's always just two eye holes with no whites. In the case of Spider-Man, whenever he takes off his mask, the large eye-white area is still there, so it is more an integral part of his mask---even when it is not being worn--than it is for Batman. And yet, somehow his mask's eye whites follow his expressions when he is wearing it...again, something that they have not been able to replicate in live action.
Power Rangers isn't exactly great acting by a long shot, but part of the reason why they had to do over the top gestures every time they spoke (besides "looking cool" for the excitable kids watching) is that they had to compensate with their bodies for the expressiveness that was lost by having a fully covering face mask/helmet.
When your mask covers all (or a lot) of your face, there is only so far that acting skills can take you without being able to use your eyes for expression. Even Willem Dafoe, as good of an actor that he is, couldn't get his emotions to penetrate that Goblin mask and needed his eyes to be visible in some scenes.
stinky pooch instead 
