Wow
What a topic.
In my years as a dive guide I saw many divers I would liked to have jerked their cards but sad to say I couldn't,due to either bad bouyancy but mostly harassing the marine life (puff up puffer).
The divers I watched having bouyancy prob's most of them were the "I took a class a few years ago crowd"
I would take them aside and work with them on the next dive, the others well what can you do.
Then there is always that "How deep can I go here" Cowboy Diver, reguardless of the boats dive plans.
A few years back some of the cert agencys talked about expireing c-cards and the diving public had a fit "just another way for them to make money" was the cry. So it was dropped in favor of the refresher class.
It's not the training agencys or your Instructor that sets the standards for a level of training, that is/was set by the RSTC.
www.wrstc.com
Back in the day the different agencys taught what they thought made good divers. That might have included pushups with your gear, ditch and don or even student diver harassment like shuting off your air for no reason.
Then they got together and formed the RSTC so a NAUI, SSI, PADI card and so on reflected the same level of skill training.
It is up to the training staff, Inst.& DM's to make sure that the student not only can do the skill but is also comfortable doing the said skill.
Then it becomes the divers responceability to decide when he is out of practice, and take a refresher.
But refreshers are just a way for the LDS to rake in more money I have been told.