I agree on parsing/serialization/persistence; but when interacting with a new JSON format, although the parsing is free (via a library), you still have to interpret/bind/integrate it into your own app's data structures. Often this is implicit in looking up values in the JSON, then calling something with them - but there's still that intermediate layer. If you're often dealing with new JSON formats (and I think most developers aren't), this integration/gluing would take up a large proportion of development time.