Let's not forget that the Babylon 5 computers are pretty advanced.
They recognize Voices and Faces.
Anyone Not programmed for clearance to secure areas would be noticed by the security protocols.
And, just because we have never Seen them closed, doesn't mean there Aren't any doors at these checkpoints.
The doors may be programmed to stay open all the time except in emergencies, in case of a security problem (i.e., the wrong person in the hallway) or if ordered closed by someone on the command staff.
As far as a door staying open while twe people finish a convesation, if the computer can recognize an order to Open in the first place, it can be programmed to wait until the room's occupant has finished talking to the departing visitor.
In fact, we have seen the doors Wait until told to close when someone leaves which would indicate that the Wait routine isn't quite sure if it should close.
Notice, however, the doors to people's quarters do not open via Voice command from Outside. They need a keycard or a keyed lockcode to guard against recorded voices and imposters.
A couple characters even get told by friends that they need to change their lockcodes more often.
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The 3 most common elements in the Universe:
Hydrogen, Greed, Stupidity!