Implementing this API would allow 3rd party apps and widgets to rate the songs at the request of the user.
One use case that comes to mind, is when I'm listening to Spotify radio on the go and controlling playback with my Pebble smartwatch. I would like to thumb up a song without pulling my phone out of my pocket. The dev of the music controller application (called music boss for pebble) has already started using this API for apps that have implemented it. Right now there is only one, Google play Music.
Please don't let the competition get ahead of you, implement this API.
There is plenty of documentation for this API over on android developer page under the Ratings for Remote Controllers section.