I fail to see the use case too. If you want a compact wire format, there are numerous options (protobufs, avro, thrift, etc) and their size is achieved by storing the data in a compact binary format and the schema separately.

With MessagePack the schema is embedded alongside the data, exactly like JSON. So it's just binary instead of text encoding, which saves some space, but as you pointed out, standard text compression algorithms are going to likely perform similar or superior.