I dont know about who should or should not be on the flight deck, but never forget the travelling public's perception of flight crew. The majority of us who work and travel in the industry probably couldnt give a rats ass whether the crew are dressed in a morning suit or just their underwear, as long as they can do their job to the best of their ability we're generally happy.
The majority of other passengers who aren't quite as savvy, expect the crew to be smartly dressed and look the part. Even if you've had a long day, got lipstick on your trousers, etc. you have an aura of professionalism to maintain, and it doesnt look good if you wander around with your pants down.
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It wasn't me.