I've used it to store blobs of data in databases, specifically when space tends to directly equate to memory (like redis and mongodb). You can take this pretty far and apply different serialization or compression algorithms based on the data (and store a field that says which approach was used for when you deserialize it).
Using this from the browser is not the first thing that came to my mind.
Storing a message-pack serialized object in MongoDB is silly, because Mongo is essentially storying BSON-serialized object on disk. You're double-serializing data in two competing formats.
BSON isn't meant to be compact, it's meant to be quick and efficient to serialize and deserialize. You store it inside MongoDB as bindata, and you'll save space.
Using this from the browser is not the first thing that came to my mind.