WordPress XMLRPC is widely used by third party application to communicate with WordPress sites and do almost everything without opening the site or even the admin panel. You can read posts, write new post and even update your plugins, activate, etc. I had earlier developed a WordPress plugin (Kish Multi Pro) which was used by 1000’s for managing multiple blogs.
WordPress is moving towards WordPress REST API which can do similar things. I recently started working on this and developed a simple theme using Framework7 which is full featured HTML framework to build iOS and Android Apps. You can check out this site on your iPhone or iPad.
Using REST API, you can pull information from WordPress installation as you can do using XML-RPC. Many applications have started using the REST API and as per Matt, this would be the future.