It's narrow-minded to assume that movies based on games won't work out, it can be done well if done right. Many of the directors so far simply didn't do it right, some even approach a game without the slightest grasp on why people like the game that could fit into a movie.
But personally, I think it's bad idea to do a video game movie with the plot entirely based on a game, A video game movie that carries some decent originality while keeping the essence of the game is what helps the mainstream understand why the fans like the game, while allowing the fans of the game to enjoy the movie to a similar degree.
Also, personally, If I were to do a Mass Effect movie, since the game francise is already well story-oriented, I wouldn't base the movie directly off the plot of any of the games, but I might introduce another chapter into the Mass Effect storyline like the guys that did the Mass Effect novels did.
One could base the movie on a major battle of the Krogan Rebellions, or anoter story on the adventures of a Alliance or Cerberus soldier like the games did with Shepard or Jacob, or something like that.
think that would be a good approach for a Mass Effect movie?