While I haven't benchmarked JSON vs protobuf, I've observed that JSON.stringify() can be shockingly inefficient when you have something like a multi-megabyte binary object that's been serialized to a base64 and dropped with an object. As in, multiple hundreds of megabytes of memory needed to run JSON.stringify({"content": <4-megabyte Buffer that's been base64-encoded>}) in node
JSON is the default serialization format that most JS developers use for most things, not because it's good but because it's simple (or at least seems simple until you start running into trouble) and it's readily available.
Large values are by no means the only footguns in JSON. Another unfortunately-common gotcha is attempting to encode an int64 from a database (often an ID field) into a JSON number rather than a JSON string, since a JS number type can lead to silent loss of precision.
A more thoughtful serialization format like proto3 binary encoding would avoid both the memory spike issue and the silent loss of numeric precision issue, with the tradeoff that the raw encoded value is not human readable.
Isn't HTTP POST content similarly encoded? Likewise with small embedded images in CSS, though I am rusty on that topic. Likewise with binary email attachments in SMTP (though this may be uuencoded, same net effect).
The particular example of a trivial message that is mostly-binary just sounds like a useful test case, more than anything else.
