Do you think a way for flightdeck crew to look in the cabin using cameras improves safety
Yes because thats where the terrorists are likely to be hiding. It also allows us gauge how close to securing the cabin the crew are.
Vice versa (cabin to flightdeck)
Not at all. Unless they happen to be trained and rated on the aircraft type and have been watching us for a long time they'll have no idea what we're doing so whats the point? If they don't get a response from us within an appropriate timeframe then there are perfectly satisfactory back up procedures in my company.