A) It's not been updated in ages
B) It's a no brainer that this should be built into the core.
An API would allow:
- Very easy integration into 3rd party software
- Infinite customisability if the API was to allow addons to 'hook' into it to add extra API classes
- Modding would be stupidly simple to do
A few usage examples:
- Mobile apps. Creating a complete native Mobile application would be possible without having to resort to calling to webview and displaying a 'dumbed down' web version of the system.
- Remote user authentication by calling direct to the authentication system without having to reload or redirect people
- Remote reply posting (ideal for blogs, you could use XenForo as your comment system)
- Automatic thread creation (again, a good one for blogs)
- Integrate into CMS systems
- Pull any sort of info out of XenForo