Not every airline has in-seat entertainment systems like Air Canada does or has plug ins for your laptop like they do. As a result, it is often a matter of having movies with me on my iPad or having none at all. Plus even when I am on Air Canada, there are often no movies that interest me and I will have seen all the ones that might be good. When you log as many miles as I do and they only change movies once a month, you can go through their listing pretty fast.
So yes, it does take some time to load them into iTunes, but it is worth it to have entertainment with me on the airplane.