I’ m not sure if I have to interfere, many nice stories are told this way, but all the answers had nothing to do with the original question! Why the captain in uniform was refused to board as a pax.
Well, I believe, there is only one answer.
In some companies, where rules are stricter then in others, in case of emergency they don’t want to leave any doubt that the captain is seating in the cockpit and not, maybe, drinking in business, or even in coach.
In a smokey situation is very difficult to distinguish the real captain in uniform, if you can see two of them, or only the pax one.