Was asked for my AOW card when diving with Buzos De Mexico in Isla Mujeres. Very good operation in my opinion. I know that in litigious countries like the US, dive ops just want to avoid possible lawsuits due to a customer's incompetence. That said, just as others have previously stated: no certification guarantees that knowledge was learned/remembered, but I guess something is better than nothing. As far as non-litigious countries, I'm guessing some dive ops want some assurance that customers won't be a liability in terms of screwing up the dive for other competent divers.