While DHT is indeed decentralized, a central tracker is still preferred since it can quickly give an authoritative list of peers (rather than a probabilistic one).
Private sites tend to disable DHT anyways, since it's much harder to detect cheating without central trackers.
The important thing is that trackers are easily decentralized, since they're a per-torrent URL.
Comments
While DHT is indeed decentralized, a central tracker is still preferred since it can quickly give an authoritative list of peers (rather than a probabilistic one).
Private sites tend to disable DHT anyways, since it's much harder to detect cheating without central trackers.
The important thing is that trackers are easily decentralized, since they're a per-torrent URL.