I know it happen a lot and will be in this way for ever...
but what I believe, that is not a Standard Procedure and not very Safely.
Weather is good, 3 CPT in cockpit and all you want...technical malfunction or failures, did not advise you..and could happen even in the best ever day of the year...
And...
..she/he will be put of the seat in No Time
I don't believe so..what do you think could happen if they experience a fast decompression or worst case scenario an explosive decompression..
believe me is better to stay every one at his own seat! and to have time to do just they should have to do by hearth instead of loosing time to remove somepne and get his seats...In aircraft there is no time in case of emergencies!
Time is the main factor who decides life and death
I used to fly "relaxed" but believe me every time I stay alone in Fligh Deck I always review by my mind some emergencies could happen to me at that time just to be prepared to handle all alone..have no time to wait the CPT....unless in paradise