EDN [1] has one flaw. It is not JSON. I will just repost the response [2] I received when I mentioned wanting EDN support in PostgreSQL.
> So the same argument as (say) YAML, Lua tables or TNetStrings. Or, if you include binary representations, stuff like Thrift, XDR and ASN.1. You can embed a JVM in PostgreSQL and write the functions yourself (or use PL/Scheme I suppose), but I would be basically amazed if anyone decides to do it for you. JSON gets the nod because it's understood by billions of systems. Other formats are going to struggle.
While I do not agree with his position, it is shared by many people.