I think your attitude to this depends on where you come from and where you are flying. If you are working for an established European carrier whose cabin crew predominantly all come from the same country and which has a low staff turnover you probably wouldn't see the problem. You are on the same cultural wavelength as the people on the other side of the (locked) door and they can all be (and probably are) properly security checked. I used to work for an airline like that and if I was still there I would probably be wondering what all the fuss was about.
However, there are airlines implementing this policy who hire hundreds of temporary cabin crew every year who get minimal training and minimal security vetting. There are other airlines that have such a high turnover of crew that they hire thousands of 21 year olds every year from all over the world including countries with known links to terrorism. It is not physically possible to security clear that number of people from that many countries properly. In the past, of course, these people still had access to the flight deck but it was fairly rare for them to be there with only one pilot. Now is is a certainty. That is why we need to ask, who watches the watcher?
Also, it used to be policy in my company that the flight deck door was opened as seldom as possible. With the new two person rule it needs to be opened twice as often as it was before and it wouldn't take a genius sitting in the first fewrows to spot the pattern. This becomes a bigger problem if, as on many types, there is a passenger toilet right next to the flight deck.
This is a badly thought out policy introduced as a "must be seen to be doing something" reaction to an extremely rare occurrence which leaves us all exposed to much greater risks much more often.