The downside with downloadable movies is that I can't burn them to a dvd. If I buy a song on iTunes, I can burn it to a CD to listen to it in my car or CD player at work. If I buy a movie, I can watch it on my computer or buy an Apple TV to watch it on my TV.
There isn't enough portability. If I want to go to a friends house and bring the movie, I can't just burn it onto a DVD and bring it, I'd have to tote my computer and cables to hook it up or an Apple TV or whatever.