“… if I haven’t read the source code or the documentation, why would I want to call this function?”
Exactly right. I have never–ever!–made an API call without looking at, and fully understanding, the detailed description of the function. The function signature with the type information simply is not enough. I need to understand the semantics of the parameters and the possible data values that they can assume.
For this reason, using a statically typed language is not important for me. Never has been.