[rybosome]

Performance is a nice benefit, but the standardization of message passing is by far the biggest benefit in my opinion. Within a given language, I know that any API I call will have certain unvarying semantics, I can see a highly readable yet formal spec of the data being exchanged, and the code to manipulate these messages will always be familiar and idiomatic.

Duplicating these benefits with XML or JSON would require defining your own grammar and parser, but wouldn't have the performance benefits. Recreating the performance gains would require a new serialization scheme, at which point you'd have broken from JSON and XML standard tools and recreated protobufs in everything but the proto definition language; at that point, why not create a DSL rather than bolting this functionality into an existing one?