The main problem with asking people to show their PDGA card is that many people don't want to openly hand over their PDGA password to anyone. Until you take the PDGA password off the front of the card, many people will continue to think of the card as a matter of something which can breach personal privacy. The other problem is that many simply don't carry it.
We have never asked for cards because it just isn't practical, however the pre-printed info for new supporting members is much appreciated.
Bottom line, we don't want to eat $10 per person who mistakenly thinks they are PDGA current.