The XML format is not there just for the sake of having an XML format. Mac OS X provides not only a parser for XML files, but also for the specifc XML format that plist files use.

A plist is translated into a tree-structure of object (hash tables, arrays, and various wrapper classes for strings, integers and such). It's used for pretty much everything in the system. In yet another callback to the UNIX-complaints thread, this is the sort of facilities an OS should provide, to free developers from the burden of creating new data file parsers over and over again.