How about the newly qualified local dive guide in the Red Sea or Thailand, checking your credentials upon arrival?
I mean honestly, we are all making mental notes of the entire situation not focusing on any one item alone like a log book or card... at least I do...
Sure. Unfortunately, this doesn't go for all the outfits out there, whether it's Hurghada or Cozumel or Corfu. These are the places where funny initial decisions may well be based - in fact, often are, at least by some staff - on your plastic c-card.
Me, I can't always tell the difference between diver A in a silly t-shirt from Belize with a card from agency A or diver B in a silly t-shirt from Florida with a card from agency B. Sometimes these geezers set up their equipment exactly the same way, and both talk the talk ...
If in doubt, I usually go for the Italian super model or the Winona Ryder lookalike.