In Sydney, a movie DVD costs about $US15 to $US22. A rental for a week costs about $US3 to $US5.
Therefore, unless you intend to watch the same movie on, at least, 3 separate occasions - each spaced at least 1 week apart - it makes no sense to buy that movie. But who actually watches a movie that many times?
Therefore, it makes no sense to buy movies instead of renting them.