This plugin disables XML-RPC API in WordPress 3.5+, which is enabled by default.
No. This plugin is as simple as XML-RPC is off (plugin activated) or XML-RPC is on (plugin is deactivated).
There are two easy methods for checking if XML-RPC is off. First, try using an XML-RPC client, like the official WordPress mobile apps. Or you can try the XML-RPC Validator, written by Danilo Ercoli of the Automattic Mobile Team - the tool is available at http://xmlrpc.eritreo.it/ with a blog post about it at http://daniloercoli.com/2012/05/15/wordpress-xml-rpc-endpoint-validator/.