I agree that it's potentially useful, but it also adds it's share of complexity to any implementation I've come across. And most proposed uses I've seen do not

I agree that it's potentially useful, but it also adds it's share of complexity to any implementation I've come across. And most proposed uses I've seen do not obviously benefit from the idempotent approach, from my experience it often complicates the server implementation. I've found using the REST approach for URLs, separating reads from writes and using status codes a good compromise.