^Amin's post points out something that I think many people don't consider enough when looking at fast lenses. Control over DOF and the ability to easily throw backgrounds out of focus is nice to have, but sometimes you need more in focus than that f2 aperture on FF gives you and that is a big advantage of smaller sensor cameras with fast lenses, especially with the increase in high ISO performance with the newer sensors. I agree with Ray that its different cameras for different uses and its a good argument to have different sized sensor cameras in your bag as opposed to just two FF bodies, or two m43 bodies for example.
Agree. And I think this is why many of us have at least 2 or more systems.