Well, a segmented (or as i like to call it, parallel) download really just means that YOU are getting your file quicker, than if you had to wait on one host. If you then allow the file you just got to be shared, the network has grown that much faster.

Your one file might not be much, but if many ppl are doing it, a given file would be replicated in multiple locations more quickly. The more locations that have a file, the easier it is for somebody else to download it in parallel, further increasing replication.