Simplicity is good, but so is flexibility and compatibility. As I mentioned, parts of REST aren't good enough; anything that doesn't have access to http status codes won't be fully compatible. A decent RPC protocol like JSON-RPC is both flexible and simple. An added bonus to not marrying your protocol to http is that your API will work seamlessly over other protocols should the need arise. I have an API that was originally intended for web clients, but later the need arose for some internal tools to be able to call the same methods remotely and asyncronously. Instead of going over http, the jobs are submitted to gearman in JSON-RPC format and everything works without having to modify the server or unnecessarily and inefficiently route the calls externally through apache.