[asperous]

Magnet URLs can and usually do include all of the trackers.

http://www.bittorrent.org/beps/bep_0009.html#magnet-uri-form...

That being said DHT and PEX (BEP 5 and BEP 11) is good enough that you don't need them.

[anacrolix]

That's not entirely true. DHT is generally the most effective, but PEX and trackers help a lot when there aren't many seeds, or there's a lot of noise on the DHT.