I've mentioned J's terseness as an advantage for codebase using little context room in agent/harnesses.
A new advantage is deterministic code to assist LLMs/harnesses. A popular new category of models is fast cheap classification (Jev is first breakthrough), which can quickly solve 1-50 classification questions about a prompt/text. A deterministic program can analyze the collection of classification results to produce a final decision.
A big J advantage is that its cold start is much faster than python or node/JS, as well as fastest program load. That a J function to grade one question "automagically" works with a list of questionss is another advantage.
As always, I'll be availabe at the end of the session if there is time available.