They probably aren't leechers--they are part of the "download mesh" just like you, i.e--all the users who are sharing all the pieces of that file they have as you all download it. The advantage for you is that if one of them gets a part of the file you don't already have, you'll be getting it from them very quickly. This great idea is called Partial File Sharing (PFS), and solves a big leecher problem. The original source uploads parts of the file to different people who turn around and share their part with others. Neat, eh? No one user has to take all the hit on bandwidth and slots.

You can turn PFS off in the options, but that's pretty cheap.

Cheers