I am not saying that you have to test every single piece of your code, but IMO sooner or later you will be forced to do so. Weather it is through the type check

I am not saying that you have to test every single piece of your code, but IMO sooner or later you will be forced to do so. Weather it is through the type checker (static or gradual), unit tests or manual tests is up to you. Unit tests just make this a little easier. You can replicate unit tests every time you commit (testing for bugs) without writing them down.