Very good question. It seems similar to the problems of translating a story into language that may not be literal but true to the feeling of the original. Sometimes you see a person screaming at another obviously frightened person, and the subtitle says "you are stupid" or other such unhelpful statement. Or the people in the movie are speaking idiomatically and the subtitle gives the translation of the speech but without getting at the underlying meaning. Sometimes it would be better not to have any subtitle at all. In the first example just watching the play of emotions is much more expressive than a straight translation.
In the comedy, "Silent movie" directed by Mel Brooks (as I recall), there is a very funny place where you can watch the lips of the actor saying "you goddamn son of a bitch" and the subtitle says "You bad boy!" That movie is worth watching and not just for the story. The medium is the message.
No problem, Himdad! I love to compare the subtitle to what I see on the screen. When I see a funny one, it was suddenly worth watching an otherwise boring movie! I don't mean to say that all movies with subtitles are boring. When it is done well, it adds to the movie for me.