Read this:
http://www.gnutellaforums.com/showth...&threadid=5476

It’s not a tactic to make you share, it is just an over active safeguard to prevent you from getting a corrupted copy of the file you are downloading.

Xolox lets other users download the “partial” you already have, so that you both don’t have to compete for a slot with the primary file holder. When download a popular file, some of the data you download, is likely coming from someone who is still in the process of downloading it themselves. (E-Donkey does this too.)