(Formerly a FFQ Article)
Anti-heroes provide an alternative to the run of the mill heroes in fiction. While your typical hero grows and changes throughout the course of the story, anti-heroes are flawed, and the flaws are usually permanent within the story. Anti-heroes are usually defined by their flaws while the typical hero must overcome their flaws. The anti-hero provides a hero that can overcome obstacles in their story by means that would typically demean or lower the status of a regular hero. It allows the anti-hero to straddle the line that separates the light from the darkside. Riddick, in the movie Pitch Black, is a perfect example of an anti-hero. An imperfect hero is always more interesting, add the hero that has a far more loose moral compass, and you have a hero that possibly has no barriers to prevent him from succeeding.
Anti-heroes are always more fun.