TDD and app building

In a post on the value of TDD, Sean McGrath says this about dynamic languages on the stiff VMs:

Jython : Anybody coding in a statically compiled programming language that is not using a dynamically typed programming language for the tests, has missed a golden opportunity for a productivity leap. In the C world, CPython. In the .NET world, IronPython. In the Java world, Jython.

If your productivity goes up that much, why limit those languages to tests? Why not just ditch the stiff language altogether and get a leg up on your competition?