it can help immensely to achieve simpler code in some cases. You want to have your own stub class that you manually created, that will be used throughout your test projects.
instead of creating lots of "whenCalled" on some class and returning fake values, you simply swap a class implementation with another.
you can think about it as an evolution of using hand coded stubs, without needing dependency injection techniques.
the suck typing feature allows you to only override what you care about, or to have a class that overrides methods for many different classes at the same time, in one place.
Get you now, yeah that makes sense. Still think it seems tricksy enough that it would throw most people. Guess I'd like the option to be explicit: