That would be a bad idea. The entire .torrent file would include the user's unique tracker key. And I suspect that isn't emc's primary concern. emc's probably worried that since DHT is the primary method of transport for metainfo using magnet URIs, it's useless for private torrents (private trackers and DHT don't mix). To a large extent, he would be correct. As I understand it though, metainfo can be sent even without DHT, and trackers can be sent with the magnet URI as well. The problem is that it's useless for trackers that require these unique keys in tracker URLs to identify users, as users can't share private tracker URLs via magnet URI without exposing their key, or without the URI being useless for the receiving party.