Why is the locked door there in the first place? To stop unauthorised access from the cabin. That’s it.
For those without CCTV and remote unlocking (is anyone still in that position?), I can see the need for another person. Otherwise, it adds risk.
What is the function of this extra person on the flight deck? EASA is silent on this one. As an alternative view, if you wanted to decrease risk, no non-pilot should be allowed on the flight deck unless there are at least two pilots present...