@Cagedh
Just to be clear, we are talking about the US rule which states:
if one pilot leaves the cockpit at XXX level, then the remaining pilot must don O2 mask and keep it on until the other pilot returns.
I guess we can agree to disagree, I believe this rule is sensible.
If there are two pilots in the office and decompression occurs, there are two heads trying to solve one issue. decent odds.
If only one guy is there the odds are halved, so it is good sense to have a head start.
That is what I believe.