s9y/additional_plugins — comment on commit ebe4ea94447ab4085cbd8c374b09848482ac3847
File:  line 0
By garvinhicking:

Formerly, XML-RPC was part of the s9y core. Due to ongoing security issues it was put out as a plugin, but the PEAR library was kept in place. Then an updated version of the library was put into the plugin so that people could use it because the s9y core version was outdated. It is better to put the library within the plugin to be able to update it more easily.

So I think it's better to make the plugin always use its bundled library so that it can be easier maintained and contained in itself.