I consider the line between “this could’ve been a magazine article” and “this needed to be a movie” perilously thin, which is why I run into the problem of necessity with documentaries all the time: Did this have to be a movie? Couldn’t this have been a written piece?
The kicker here, regarding Sam Pollard’s The League, is that it’s based on Bob Motley’s The Negro Baseball Leagues, so it already was literature, and Pollard found a way to turn it into essential documentary cinema.
May I suggest you read all about it over at Paste Magazine.
A Constant Visual Feast 