Amazon S3 would be a much better choice for files considered obscure. A well designed decentralized system will have to spend a decent amount of effort to ensure that the long tail files don't drop off of the network. If you want fast access speed to those long tail files, now you're going to need even more redundancy to overcome access bottlenecks.
bittorrent allows a more ad hoc approach that gives results, whereas there doesn't appear to be financial incentives for someone to pay for amazon s3 to provide the same.
