I'm not sure I agree with the sentiment that people who code in dynamic languages don't think about signatures and types. If anything, people who write in dynamic languages think about them more because they are not explicit in the language (or at least they should).

Dynamic properties of languages should only be used from the perspective of saving time - it's less work to make changes to code because you don't need to conform to a type system. I feel like more often than not it's used as an excuse to write sloppy code. You simply cannot write good code without keeping your types and function signatures correctly.