I agree with Spudracer on all points.
Movies can be very educational simply by exposing us to some event in history, social issues, technical advancements etc.
Obviously films take artistic license, so you should always do your own research on subjects based on fact or actual events.
Here are some of the films I think have some educational value:
The Tuskegee Airmen (TV Movie) - Based on real group of WWII Black pilots
Crash - Makes a hell of a social statement
Black Hawk Down - Based on a real event
The Day After Tomorrow - Warns about complacency regarding Global Warming
Roswell: The UFO Cover Up - Based on an actual event in 1947
Gia (HBO) - About a top model fighting her battle with AIDS
Milk - About Harvey Milk, a political gay activist
All the President's Men - About the breaking of the Watergate story
A League of Our Own - About a women's National Baseball league during WWII