Impossible to track with 100% accuracy, yes. But not completely meaningless to do some basic tracking. A torrent client could implement some tracking on their own, where they keep track of seen clients and keep track how much been uploaded vs downloaded from them (effectively ratio per client) and upload slower to clients you've never encountered that have been uploading.
Obviously you wouldn't (the network is decentralized, you can't know other peers communication) but also you don't care (peer A <> B might be able to upload/download, but peer B <> C can only download in one direction).
The important part is to keep track of what's happening with your own connection to others, not what other peers are doing.
> The important part is to keep track of what's happening with your own connection to others, not what other peers are doing.
99%+ of the time any pair of peers has unidirectional traffic. The situation where two clients will have incomplete torrents and are exchanging data with each other is rare, like the first few hours after a brand new torrent hits the network.
