Although the original blog post focuses on JavaScript and browsers, MessagePack itself doesn't mainly focus on them.
A major use case of MessagePack is to store serialized objects in memcached. A blog post written by Pinterest describes this use case (http://engineering.pinterest.com/posts/2012/memcache-games/).
They use MessagePack with Python which is faster than one with JavaScript. They could store more objects in a server without performance declination (e.g. gzip).
It's true that MessagePack is not always faster than JSON (e.g. within browsers), and it's not always smaller than other serialization methods (e.g. with gzip compression). So we should consider that which serialization methods should I use for "my" case.
There are also general tendency which is helpful to select MessagePack or JSON:
MessagePack is faster to serialize binary data such as thumbnail images.
MessagePack is better to reduce overheads to exchange small objects between servers.
JSON is better to use it with browsers.
A major use case of MessagePack is to store serialized objects in memcached. A blog post written by Pinterest describes this use case (http://engineering.pinterest.com/posts/2012/memcache-games/). They use MessagePack with Python which is faster than one with JavaScript. They could store more objects in a server without performance declination (e.g. gzip).
It's true that MessagePack is not always faster than JSON (e.g. within browsers), and it's not always smaller than other serialization methods (e.g. with gzip compression). So we should consider that which serialization methods should I use for "my" case.
There are also general tendency which is helpful to select MessagePack or JSON: