I'm not a Node dev, but I've done some work with JavaScript on the web. Why are they using JSON to send data between v8 isolates when sendMessage allows sending whole objects (using an implementation defined binary serialization protocol under the hood that is 5-10x faster)?
The biggest reason is most likely that they don't understand serialization, and simply think that objects are somehow being "sent" from one worker to another, which sounds like a cheap reference operation.