Valid point, and PUT would indeed be appropriate. But are there any benefits to this approach? I'm sure there is an edge case I can't think of right now; as I see it, there is one more trip to the server, plus extra code to accommodate it versus a simple POST to a collection endpoint.
Not all resources are part of a collection, though. It can be a resource linked from another. Let's say 'product' resource you're selling. That may return a bunch of data, including a link to a 'description' resource about the product.
Since you've never created such description, the link will return 404, but you can PUT a description to that URL without having any extra trips, just the visit to the product.
never had the call to do it myself. Just something I keep in the back of my mind for a rainy day. The benefits would be application specific, I don't think it's something that you balance against POST to the collection in the general case and do one approach exclusively. I have used POST to collection extensively and PUT for create never. One example I can think of is you are making one of those sign up forms that does AJAX to check that the username is free or not. You could use a PUT in a case like that because you've already done the extra server trip as part of a usability improvement.
