You have it exactly right, I think: Lisp's advantage is the interactive repl environment. I do a lot of work with NLP and keeping a (SBLC) Lisp image with all required data loaded saves a lot of time. Even if I am not working in the mode of keeping pre-built images around, starting up an environment and not having to restart it saves a lot of time. I really love to code in Ruby, but the repl experience is better in Lisp.