[S4M]

I don't have the exact link, but I recall an article on HN where the OP complained about NASA stopping to use LISP. The killer feature LISP had was what is just described here: the possibility to interact with live systems through the REPL - in this particular case, it wasn't a web app but a space probe. With it, you could simply fix some bugs while the probe was far away - orbiting around Mars IIRC, while you wouldn't be able to reprogram the whole probe system.

So, there are cases where you want to use the REPL to modify live systems in production.