'Real Swarming' does indeed exist, in Edonkey2000. Partial files are used immediately for uploading (once one chunk is complete), and upload priority is given to the rarest chunk that you have.

The result is that even HUGE archives can be uploaded to thousands of people in just a few days. Take for example the 7 CD archive of a certain giant software company's latest visual development tool. We're talking about over 2.5 GB of data (7 CDs were not all full), released as 7 image files.

Starting just 2 days after this download was announced on an Edonkey2000 forum, it took only 1-2 days to download everything. That is the advantage of real swarming - the distribution pyramid that forms to spread files happens as quickly as a single chunk can be spread. What users see is that hot new releases are easy to download right away, not weeks or months later.

So, in answer to your question:
"I wonder what percentage of people never download a 'commonly downloaded' file?"

The answer is that everyone who ever wanted a 'hot' release of some new large file, like a movie or game, were in fact asking for a 'commonly downloaded' file. That's most people, I bet.

Oggs