Indeed, Mac OS X ships with a GUI program to edit plist files quickly. Of course, this is seldom necessary, as programs usually have proper interfaces for editing settings. But when you need to do some low-level hacking, it gives you a quick way to look at and edit not just any config file, but lots of other files too - most data files are plists, too.

Arguments about code size are silly. If you based all decisions on what to include in an OS on the requirements of embedded systems, we'd still be stuck in the seventies. If it is really too much to keep an XML parsing library around, then you make alternative lightweight tools for the embedded systems. You already mentioned busybox, which is exactly that. By your arguments, we shouldn't be using glibc att all for anything, because it's too big and bloated to fit on embedded systems. I work with embedded systems, and let me tell you, Linux itself is far too big and bloated to fit on those I use. Let's never use Linux anymore!

Here's a question, which produces more bloat: One XML parsing library, or the config file parsing code in every program that uses a config file?