If you're downloading the movies with an internet connection, there isn't really a good reason to delete them any more, unless you're more worried about copyright police raids than archival preservation; http://lists.canonical.org/pipermail/kragen-tol/2010-October... shows that the cost to buy a new disk was only 20% of the cost of paying for the bandwidth. Now it's probably more like 8%, and that's here in Argentina.
I don't pay for bandwidth, I pay for an Internet connection, and since disks can't replace most of its functionality (like reading HN every day), I'll still pay it regardless of whether I download the movie or not. Therefore, downloading a file is effectively free as far as bandwidth costs are concerned.
This would be different if bandwidth was metered, of course, but at least here in Portugal it isn't.
Sure, downloading a file is effectively free; there's no marginal cost. But buying a new disk instead of deleting the file is almost free. If you stored every byte that came down the pipe on a disk forever, it would only increase the cost of your internet connection slightly.
