If someoens getting it just so they can upload at 1KB/sec instead of 30KB/sec. That's evil and should be punished.
As long as the source is out then there will be people who will alter it. I'd not be surprised if someone altered a leech version. :\ Which is like defeats the purpose of file sharing. Kinda like the people on Kazaa with no shares or disabling uploads.
I was thinking of a away around this. Most people will play fair and use the official client. But I am sure some out there would enjoy not having to share. The official client could have peer checking. Where it would ask a peer where it sent a chunk to and how long ago, then it would ask that peer if it received that chunk. If not and if this happens multiple times, or if there is no activitey and there are peers that need bandwidth then that client will get kicked off the network. Keeping them off though would be tough. Since it's a public network if added banning to the protocol, then anyone could send out a ban msg and kill a entire torrent just for fun :\
Anyways

~klinky