But hstore will be faster/better because it's a binary representation, right?

JSON will be getting binary representation. Its just a matter of organizing sponsorship of the project.

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.

I'd recommend reading the recent post https://news.ycombinator.com/item?id=6813937

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.

Very likely so for the majority of use cases, yes.