Work around for some versions of libxml2 2.7.x that strip out brackets when parsing XML. This plugin fixes XML-RPC requests that are mangled because of this problem. The real fix for this (making the use of this plugin unnecessary) is to use PHP 5.2.9+ with libxml2 2.7.3+.
For more information about this problem see https://core.trac.wordpress.org/ticket/7771.