I assumed everyone would be encouraged to just convert their json columns to hstore if they wanted the extra speed, being as theyre equivalent. What would be the reason for funding JSON-as-binary?
For better or worse, JSON is becoming a major format and it is a first-order Postgres data type. You need to add the hstore extension if you want it. So it makes sense for people needing/wanting JSON and are not aware of some extension called hstore. In other words, it is about improving the ease of use.
Traditional databases use complicated indexing and storage structures in part
to minimize the need to examine unrelated data. hstore is better than plain text
for JSON but a scheme closer to the existing PostgreSQL table/page/row/value hierarchy could
be better still.
Of course if I need it to be eye-wateringly fast I wouldn't be using a document store. I'd use good old relational or something like redis. Often though you just need a sensible place to put some unimportant schemaless key-value data without a half dozen extra dynamically created tables or whatever. EAV makes me very sad.