I use auto-completion in Python every day with PyDev. It's not as advanced as for other languages, but "just can't work" is not really true either.

http://www.pydev.org/manual_adv_complctx.html

That also depends on the namespaces used by the language. E.g. auto-completion in Clojure works a lot better than in JS, because every symbol has a unique resolution, known statically (or as unique as in many typed languages). True, you get less helpful suggestions, but OTOH, the error messages can be much clearer than in the typed case. The former can leave error reporting to the DSL, while the latter is restricted to the cryptic types of the host language's compiler. I'm not saying this can't be resolved with some clever compiler plugins, but if we're comparing options available now, the advantage is not so clear-cut (I do favor types, but only as long as they don't get too complex).