Quite. If you lock the ee bay someone has to have the keys. As it is, one or more persons on the flight deck are being suspected for their involvement (whether right or wrong and I do not judge). They would have the keys anyway. A locked flight deck still lets one or more person with access to the control surfaces to do things they shouldn't do if they are so inclined. Unless you have a "drone" you will always have this risk.
Some of the initial reports wondered if a drone could have had a mid-air with mh370. I know nothing about drones - are they so big/can access that Fl and really create a total loss?