Sybil attacks are like DoS attacks. They are never going away fully. All webservers are vulnerable to DoS attacks and yet in practise it is pretty rare for major websites to go down because of them despite their simplicity.
But there are strategies to mitigate sybil attacks. Requiring hashcash style proof of work to participate in the network, increase state requirements to participate in the network, basing node id's on something the attacker cannot control (e.g. IP addressss), are common strategies often mentioned.
In the context of bit torrent, i guess it doesn't super matter that much. If your goal is to spy on the overlay network, while the entire point is to publicize the tracker data so what is the point of spying on public data? If the goal is to more actively attack the network - having bit torrent be a dual system with traditional trackers (and other ways to find peers) also, means that an attack causes degredation but is not a killing blow to bit torrent. Which in the cost/benefit analysis reduces the benefits, causing attackers to wonder if the cost is worth it.
Like denial of service attacks, the best defence is to make the attack expensive enough that it just isn't worth it.