STUN is not always possible and TURN means proxying the connection through a server which would be counter-productive for the purpose of using bit-torrent as an alternative to direct HTTP downloads as you are now paying for the bandwidth in both directions. This is very much not a problem with magic solutions.
Agreed! But STUN's success rate is pretty good! As the number of peers goes up it should be less likely that one would need to use TURN to connect to a peer, but I am skeptical webrtc is built to fall back like this, to try other peers first.
The advantage is that at least it's all builtin. It's not a magic solution, but it's a pretty good solution, with fallbacks builtin for when the networking gets in the way of the magic.