Thank you all.
Although most of your reports are in agreement with my instructor's (company policy) I still feel this is quite wrong.
I mean really, can you imagine this little 5'7 flight attendant opening a door and looking at a burning RB211 spewing fuel/oil/hydraulic fluid saying to herself.... ''Nope, that's a bad exit'' and now goes into exit blocking procedure that is nothing more that standing there blocking the exit with her body, and shouting : ''do not use this exit''? and while at the same time down the isle comes charging Joe Blow... a 6''4 hulk,half drunk of all those Bloody Marys he had earlier to overcome his fear of flying, seeing blue skies through that door? he '' moves her aside gently'' and goes right out...... needles to say, he will probably not end up in the local pub having more drinks and telling stories.
What I am trying to say is this:
The Pilots will in MOST cases have a better picture of what is happening outside the aircraft.
CC will not always be able to correctly assess the external situation by peeping out the door port hole.
Once a CC had opened a door that is deemed unusable, it will not guarantee no PAX will still escape through it and will be harmed.