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.
For example, to get information about torrents that are on life support (e.g. have a couple seeds that show up for a few minutes a day), having the torrent file downloadable is invaluable, as it includes info like the size and the file names, etc. In many cases it's even possible to bring a torrent /back to life/ if you have the torrent file and got the content that was in it out of band.
If you just have the magnet link you cannot do any of this.
I use this all the time for downloading very long tail content, and the move to have indexing sites be magnet-only is very frustrating.