HTTP hosting is decentralized and not decentralized. If I run an Apache server hosting my website, it's centralized. Everyone that wants to access my website has to go through my server. They don't have to go through my server to access the whole internet net though. The internet -- for the most part -- is globally decentralized, but locally centralized.

It's the same with Bittorrent and trackers. If a tracker goes down, someone else can start up a new tracker that is serving the same torrent. It's not centralized the way something like Scour.net or Napster were (all peer-finding goes through the same servers).

> Decentralization greatly increases the reliability of the system.

Not necessarily. If you have two groups of 20 peers that are only connected to each other by a single peer, what happens when that one peer goes down? It's now impossible for those two groups to find each other. And consider that it's a lot more likely for a single peer in a torrent swarm to go down (someone's computer crashes, close the torrent, residential internet goes down) than it is for a centrally hosted tracker that is probably on a dedicated server in a colo somewhere.

PEX from Wikipedia:

  While it may improve (local) performance and robustness—e.g. if a tracker is
  slow or even down—heavy reliance on PEX can lead to the formation of groups of
  peers who tend to only share information with each other, which may yield slow
  propagation of data through the network, due to few peers sending information
  to those outside the group they are in.