Citation needed, especially since the node-msgpack library docs say it's anywhere from 1.2-3X faster. https://github.com/pgriess/node-msgpack
I've done several of my own benchmarks as well and can confirm that the current Node JSON implementation is much faster than MessagePack. Their benchmarks are most likely quite old.
There are benefits to MessagePack that have already been mentioned here, namely not having to base64 binary data first (smaller size), but that's true for any binary message format. I'd love to see some other binary formats thrown into the ring and see how they compare to MessagePack in both size efficiency and encode/decode performance. BSON seems like an interesting option, but I don't know enough about it to comment...