From my experience the main thing people are missing with BitcoinJ is
a quick and easy way to set up a wallet as a daemon, to use the
functionality from non-java through RPC.

Yes. I'd love to have a mostly Core compatible JSON-RPC frontend. Most of my current users are happy using it as a library though. A lot of popular languages can run directly on the JVM these days. The big ones we miss are C++ and PHP, I think. But you can use JavaScript, Python 2.7, Lisp, Ruby, along with other less well known ones.

The other good reason to have JSON-RPC support would be to reuse the Core regression tests.

Anyway, this is off topic :)