Just because you bought a movie doesn't mean it will stay in the cloud forever. If the movie isn't for sale in the iTunes store, it will be pulled from your purchases as well and you won't be able to download it again. An example is Poltergeist 2. I bought it probably a year ago when it was a $4.99 movie. It got deleted from my computer sometime over the year and I went to download it again. It's not there. I look in the store and it's only for rent so I can't download it from the cloud.