Low bandwidth users don't use swarmed downloads by default; only high speed ones do (unless the client is poorly written). Your point is well said, and low bandwidth users cannot benefit when it comes to download speed. However, it does help them when uploading. People with faster connections can download small chunks from modem users. This way, modem users can share without being penalized for it. This would encourage them to share and not be a complete freeloader.

:-)