In my view, GWT is for creating large/robust web-based applications, and that is where machine-level optimizations really pay off. If you are writing a tiny utility on a page, hand coded javascript might pay off, but if you are writing a large/complex application, it becomes impractical for a human to do what a optimizer can.

I am not saying that GWT is perfect for everything, but it pains me to think of writing (and debugging, and refactoring) a large application in javascript, or any weakly typed language. I've tried it, and found it difficult to maintain.