You have to say that 'Captain John Smith ATPL (OATS)' has that certain ring to it. Unfortunately, under the new EASA order, the licence is now known as 'Part-FCL' and we can all see what putting
that after your name would lead to, can't we?
I believe that only holders of the Queen's Commission are entitled to append their rank to their name, strictly speaking.
All this sort of thing went out with Senior Captains (First and Second Class) in the old BOAC and BEA, and the wearing of medal ribbons. Indeed, opinion was divided in my last company about the wearing of epaulettes on one's uniform raincoat. I suppose you can call it inverted snobbery or the pricking of pomposity in these CRM times.
Oh yes OATS ... Oxford Air Training School.