[Arkadir]

Easy language interoperability as a reason to choose Protobuf over JSON ? Mainstream languages support both JSON and Protobuf equally well, and the others tend to support JSON more often than Protobuf.

Free backwards compatibility ? No. Numbered fields are a good thing, but they only help in the narrow situation where your "breaking change" consists in adding a new, optional piece of data (a situation that JSON handles as well). New required fields ? New representation of old data ? You'll need to write code to handle these cases anyway.

As for the other points, they are a matter of libraries (things that the Protobuf gems support and the JSON gems don't) instead of protocol --- the OCaml-JSON parser I use certainly has benefits #1 (schemas), #3 (less boilerplate) and #4 (validation) from the article.

There is, of course, the matter of bandwidth. I personally believe there are few cases where it is worth sacrificing human-readability over, especially for HTTP-based APIs, and especially for those that are accessed from a browser.

I would recommend gzipped msgpack as an alternative to JSON if reducing the memory footprint is what you want: encoding JSON as msgpack is trivial by design.