A truly "open" game

I've been thinking about building a MMO of sorts for the longest time, and during one my many "thinking sessions" I came up with this question: "Has anyone built a game where it is purely an API and People create their own clients?"

In a nutshell the game would exist as a REST endpoint, that exposes a number of APIs covering all the possible interactions with the game. Here are some examples:

• user signup - Register a new user
• start quest - Initiate a quest
• end quest - Checks for end quest criteria, awards items/xp etc...

Obviously a great deal more throttling/validation would have to be employed to prevent cheating, but it would be no different than any other web service.