Syndirella has a good implementation of RSS feed auto-discovery and this is where Dmitry uses XML-RPC:
My current goal is to be able to discover automatically the feeds for all sites which I regularly read (and which do have feeds). And neither Kuro5hin.org nor Joel on Software have feeds which can be discovered by links. So I need to fall back to the last option in Mark Pilgrim's autodiscovery scenario - search Syndic8.com. And this requires XML-RPC.