Stallone doesn't always have the helmet on because...

...the movie is about Dredd having a twin brother. You'll notice there's no shortage of direct shots of Dredd's 'blue' eyes. It's integral to the plot. I know it really upset fans of the comic book, (but honestly what doesn't upset fans of any comic book?), however, it was a good way to form that connection between the 2 main characters and have the final climax and plot reveal make sense.

The helmet thing was way overblown, always was. This is still the best Judge Dredd film between the 2 movies. This 1995 version has good character building and some good drama particularly revolving around Dredd's past. The 2012 version is completely mindless regarding the plot, but it has good special effects and awesome action scenes, but there is no payoff and zero drama.