Karl Matthias, an engineer who writes about Go, Rust, Elixir, Cassandra, event sourcing, and other backend systems topics.
HN links his Cassandra versus MongoDB comparison as a reference when discussing database migrations, and commenters remark on his early Rust posts.
Can you give some context to your benchmarks because it looks like it's comparing messagepack to json in javascript which is one of the few places it would be slower than json. Since the json parser is implemented native vs message pack being implemented in js. Every benchmark I have seen in other languages where json and messagepack parser are on equal footing shows messagepack being faster and lighter. For instance on Ruby where Oj is tops in JSON, Messagepack is still faster: http://relistan.com/messagepack-vs-json-in-ruby/
Again on the server messagepack should be faster and use less CPU and require less bandwidth than json, the client will require more CPU but you have many more client CPUs than server CPUs.
Compression layered over text isn't free in any context, the closest it comes is pre-compressed static content, which would be great for the layout HTML,JMX and js files. Binary serializers like messagepack give you some space saving while also saving server CPU cycles rather than trading one for the other.
It feels like the author had a more fleshed out post in mind, but either decided it was running too long or got bored...
Or it could be that this post is just the beginning of his journey of learning & blogging about Rust. He did post another one 4 days later, albeit not yet touching on the topics you mention:
http://relistan.com/a-pattern-for-wrapping-c-function-calls-in-rust/
Here are two blog posts I was able to find from companies who switched Mongo -> Cassandra:
http://www.fullcontact.com/blog/mongo-to-cassandra-migration/
http://relistan.com/cassandra-vs-mongo/
The main selling points for Cassandra over Mongo are that it scales to more machines easier for handling larger workloads, and the write performance is better.