There are some interesting arguments in this article.
Only 21 percent of Part 91 respondents “indicated they always use oxygen when one crewmember leaves his duty station above 35,000 feet,” according to the survey; 10 percent reported occasional use; 25 percent rarely; and 33 percent never.
Thank you. The articles states that many pilots, unfortunately, do not follow the rule. Which is very worrying indeed. This does not make the rule a bad rule. It just points to how many pilots disrespectful of the rule we have out there. And that is not a nice thought.