Short answer? Grungebob said not to.
Long answer? It causes all sorts of problems having to do with keeping track of who did what, what triggers when, and that sort of thing. And for certain powers, it can get extremely confusing. If someone attacks you, and you interrupt their attack with your own attack - do you counter strike before or after that? Or at all? And if they have counter strike, when does it occur? That sort of thing...