Having access to a large repository of files without actually downloading the files to your computer already exists at large corporations, but you want a more traditional serving hierarchy in order to get acceptable read/write latencies.

Bittorrent is more useful when 1) local peer transfer speeds exceed the transfer speed from a master copy, and 2) everybody wants an identical copy of the data.

For example, if you want to deploy a new version of Facebook.com to a far away data center, Bittorrent is great (you only have to upload maybe 2-3x the size of the binary to the datacenter, and the thousands of nodes inside the datacenter can share the pieces amongst themselves).