YES! It's a pet peeve of mine. There are some good films out there, but they are the exception to the rule. Hollywood is good at entertainment, but they are not good historians most of the time. Movies, aside from documentaries, are made for profit, to appeal to a mass audience that can bring in dollars, so there is little incentive to be historically accurate. As David Milch, a well known writer and director, said of his HBO program Deadwood, "We do want to be as historically accurate as possible, but we don't want to be boring". I rest my case.
I love historical movies. I enjoy the epic adventures like "Ben Hur," "Glory," "Gone With The Wind," and many others. The idea of using these movies, however accurate we may believe them to be, as a true guide to the history of the world is dangerous. The one thing one must always keep in mind when watching a movie "based on a true story" is that they are usually not accurate. The people and corporations who make movies have two goals. The first goal is to make money. The second goal is to entertain people so they will buy tickets and merchandise. This second goal directly impacts the first goal. If in doing these two things they can tell a true historic story, they will. However, if changing some names, dates, or details of the actual event will help make the movie more entertaining or more profitable the powers that be will indeed re-write history. You can't count on Historical movies, or even all documentaries to be factual from beginning to end.
To take the information in these wonderful movies and create a factual basis of History from these can not be done. An excellent book on this subject is:
"History Goes to the Movies: A Viewer's Guide to the Best"
Yes, it is dangerous. If hollywood makes a film based on an historical event they are not obligated to adhere to the facts. Commonly referred to as 'artistic licence' the filmaker can and often does take liberties with the facts. If this is the only means an individual acquires their knowledge of history, their knowledge will most definitely be compromised. On the other hand, if an individual has the knowledge and understanding of history they will be able to take the film for what it is, a hollywood film. I must admit as an historian I view many films that utilize an historical reference for example, Gladiator, Troy, The Greatest Story Ever Told, Spartacus, J.F.K., Platoon and many others. While in the theater I always do my version of history v. hollywood, however understand that I know the history so I can take it for what its worth. The person who believes what they see in historical dramas as truth will always be on the short end of the history stick !!!
Dangerous is a rather strong word, but, yes movies are likely to take many liberties with historical facts to make their movies more interesting and acceptable to people.
Also movies have to include so many details which just cannot be ascertained historically. For example, most of the dialogues included in the movies has to be a work of fiction. Placing too much reliance on such dialogues to draw conclusion about historical facts can be highly misleading.
Similarly, the details of other things shown in the movies like furniture, clothes and buildings are frequently as much the result of imagination of the designer of these as of historical knowledge of these.
At times the film maker is forced to adopt means of story telling to convey some ideas effectiveluy and in short, which can be highly misunderstoo by viewers. I am reminded of a scene from the movie Gandhi in which Mahatma Gandhi is shown to serve tea different political leaders in a meeting do discuss partition of undivided India. This kind of meeting never tool place, but was included to show symbolically the efforts of Mahatma Gandhi do develop common agreement between different leaders through several meetings held with them individually or in smaller group.
Movies do have a positive aspect also. They help to create interest and understanding of history among a large number of people.
It entirely depends on the quality of the movie! There is no 'rule' that says learning history through cinema is bad. But like books, TV programs, conversations, lecturers, computer games, web-posts and all other methods of receiving information, it is not The Medium that is the problem, it is the quality of the content.