Static typing is a godsend when you're working with code that you only vaguely remember from the last time you worked with it a year ago. It also gives me much more confidence when refactoring: the compiler will find the callsites I missed.
I've worked on rather big projects in Ruby and Javascript, and testing always managed to keep things sane. Static typing helps you refactor things or make sure certain variables are the correct type, but in the grand scheme of things, I'm not sure type casting is the major source of bugs/errors in any system