The situation you described is why RAT or "ADG" exist. Double engine flame-out with the APU off-line. Let's put it in a simple way: If the aircraft looses all its regular sources of AC
power, is it still flyable? On the CRJ, no, as the flight controls are hydraulically powered so you need the Air driven generator to power the Emergency hydraulic pump (3B).
I'm not familiar with the ERJ, but i suppose, if it doesn't have a RAT, its
has DC hydraulic pumps, with are powered by the battery on the emergency or direct bus.
It could also be in a situation with double Ac generators going offline at the same time, for some reason. As the APU is generally not runnig during normal cruise, the ADG would deploy to power the systems while you start the APU and get the APU gen online. Wouldn't want to be in a ERJ in this situation if you can't restart the APU in flight, altough i agree these are highly unprobable situations.
It's all a matter of electrical system architecture.