You're not wrong, but I get a different feeling from "dark and stormy" than just "[i]t was a stormy night". The former is almost foreboding, and maybe suggests

You're not wrong, but I get a different feeling from "dark and stormy" than just "[i]t was a stormy night". The former is almost foreboding, and maybe suggests a particularly long night - perhaps winter? I don't get that from the latter. You can convey the same feeling in a more effective way, but I don't think it's awful.