--jp is a perfect fit for shell scripting integration:

    bash$ curl ... --jp "foo=$foo" ...

This has zero shell-quoting, expansion, escaping, or separator issues. Whatever's in environment variable `foo` will be sent to the server as a single value associated with key `foo`, whether it's a zero-length empty string, or full of backslashes or spaces or newlines or whatever.

There are fancier ways to accept multi-arg, but they all have weaknesses, and this matches the way curl handles -H arguments already today (one per header, stack them if you want many), so I think it's a sound way to handle CLI arguments.

(I don't have any specific views on whether this is how curl should do JSON or not, but I recognized the CLI safety mechanism immediately.)