If you want to merely get a general idea of a topic, then yes, I think it's a fine way to do it.  If you want to write on differences between fact and fiction (e.g. in the eyes of pop culture), then it's also acceptable.  However, as you indicated already I'd never use as a source for actual reporting on the topic itself.I can think back to when I was writing a paper that had to do with the policy and justice behind the provisions in the Compromise of 1850.  In some of my research I came across references to the issue of the Amistad, the slave ship that brought across a slave who would eventually become part of a legal decision which had an effect on the overall slavery issue in America.  It helped to watch the movie if only to bring "life" to otherwise textual facts that I had been reading about.  It didn't help me directly with the paper I was writing but it did bring context.  It's probably a good idea, though, to know which part of of a movie is fact and which is fiction before you begin watching it.