I agree that a comprehensive test suite is valuable, but I don't think it's safe to assume that errors like this will always be caught, no matter how good your tests are. It's not reasonable to write a test for every possible situation that could ever occur. In fact, it's a giant waste of time.

> of how much value is a rigorous type system?

It saves a lot of time on those tests you won't have to write and maintain just to ensure that you're getting your types correct. Strong static types are also good documentation. Some type systems (e.g. Haskell) can even explicitly indicate whether a procedure is pure or has effects. Static types also enable dispatch that can be resolved at compile-time, resulting in more efficient execution. And the list goes on.

Of course, advocates of languages without static types have heard and dismissed all these arguments before. I guess it's really just a matter of taste.