Generally speaking I would say it's mostly considered marketing but I think it mainly depends on what you're doing with the data. For TMDb, we're just trying to build an awesome movie database. Since we provide this data to 3rd parties though, we thought it would be a good idea to make sure we had permission from the studios. So we got it. For whatever app your buildinguse you're thinking about, you can make that judgement call as well.

Note that all the movie data are facts, and facts cannot under any jurisdiction (that I'm aware of) be copyrighted. "Prose" (e.g. reviews, plot descriptions, etc.) can potentially be protected by copyright, if they're significant enough to be called a "work". Images are copyrighted material though, but I don't know the details of the conditions for when you can use them and when you cannot. As long as they're used in a promotional manner, I doubt studios/actors will mind though.

Images can be used as Fair use, Copyright law does allow this. Fair use for depicting movies, characters etc..Plot/movie data in text is not copyrighted unless if you are going to recreate a movie with the same story..