Those FAs with brains are able to know if the plane just crashed, or if the flight crew is well able to observe and to act. Don't forget an evacuation is a procedure which usually causes a lot of injures, which can and should be prevented if possible.
The example 411A keeps referring to in several threads on the PPRUNE forum, from his own experience, shows a lot about why contact with the flight crew is important.
The diversity amongst cabin crew is gigantic, I've flown with the best professionals, but I have to admit, with the dummest on earth as well.
In the Lufthansa A320 rwy-overrun-accident in Warsaw the cabin crew initiated the evacuation, and saved the life of one cockpitmember by doing so. Those FAs used their possibility of initiating the evacuation in the right moment, namely where the flight crew both are incapacitated and there is an imminent danger to the lives on board.
If you see flames coming from an engine, while I'm in the cockpit, alive and kickin', PLEASE call us over interphone! It is possible we are not aware of flames coming from the engine, and also it is possible we are aware, but due to several reasons one single FA in the back will never be able to know from his/her position, decide not to evacuate. Maybe we want to taxi forward, maybe we already solved the problem and the fire will extinguish, maybe the fire crew is already standing next to us?
Sweety, 411A's tone in his posts might not be very friendly towards cabin crew, but the point he is making is very valid:
If they want to be constructive, then use the interphone.
P77