I may be taking an overly simplistic view, but it seems quite obvious to me.Who is trained and legally responsible for ensuring that the aircraft is in safe to enter and to operate? - Only the flight crew, and I suppose, taking it to the legal ultimate, it is the pilot alone.Therefore if the flight crew has not checked the aircraft, it cannot be assumed that it is always in a safe condition to enter.Likewise, as the majority of [hazard] indicators are solely present on the flight deck, if there is no-one on the flight deck, regardless of how much training any other crewmember may have had, it is incorrect to assume that even if the aircraft has been checked & was safe, that it will remain so.If Cabin Crew are being permitted now, it is only time before passengers are allowed to do the same.We have seen a fair few threads discussing fires, failures and (internal) short circuits of standby battery power sources, most caught by the attention of someone on the flight deck. Should one of these incidents occur with an empty flight deck, cabin crew on board & passengers half boarded, who will be responsible? In the worst case of a fire following a short-circuit with the aircraft as described, how much damage will occur to the aircraft before the problem is noticed & how long will it take to de-plane?This does not seem to me, to be what could be termed, a wise and considered step forward for continued safe operation.