In my opinion, the biggest benefit from using protobuf is that the schema exists in a .proto file. This can be used to provide all sorts of conveniences.
With a plain JSON-based API, you copy and paste field names out of sample code or the documentation. If you spell a field name wrong, there will be no error on the client. If you're lucky, the server might error out because it didn't recognize the property name, but it also might not. If you send an integer when the server was expecting a string, the server might automatically convert or it might not.
With protobuf, the schema is explicit in a .proto file. That means that the client library can tell you, at the precise moment that you say msg.misspledFieldName, that the field name doesn't exist. Or if you try to put an integer in there instead of a string, it can tell you about that too. Basically it makes for a tighter feedback loop, which is almost always better.
In statically-typed languages like C++ or Java, the schema can be used to generate static types too, so it's actually a compile-time error when you misspell a field name.
> It doesn't matter what what programming language or OS I am on, there is almost always a built-in library that de/serialize JSONs at reasonable speed.
Yep, that's one reason that proto3 will support JSON as a first-class citizen: https://developers.google.com/protocol-buffers/docs/proto3#j...