"It is much easier to quickly prototype or experiment in a dynamic language."

I disagree. I can say what I mean with types, and if I didn't mean what I said, refactoring is a breeze.

"You don't have to stop and figure out your types for every function and then go back and change them every time you figure out there is a better way to represent the data."

You don't have to do this in languages like Haskell either. Type inference will help you most of the time when figuring out a function's type, and you don't have to use the type system as it was intended at first. You can have a prototype with Strings and Lists, the compiler won't be able to help you much though.