RJSON not only compresses data of any structure but keeps it in JSON format that is huge advantage over gzip/7z and other binary compression algorithms. You can compress 1M of JSON to 300Kb of gzipped JSON but then you will to parse 1Mb of JSON on client anyway. With RJSON you can compress 1Mb of JSON to 500Kb of RJSON, then gzip it down to 200Kb and parse only 500Kb of JSON. Then you can analyze some core properties of parsed RJSON document and unpack it on demand. For example you may send to client collection of 20 documents, parse each and show title of each document to the client, then fully unpack only selected by client document.

I agree with Too that specialized protocol will always win, but RJSON IMO decreases structural entropy almost to 0 without need to debugging own protocol.