As a CFI, I have canceled lessons because the
compass card was missing or not legible - perhaps
the cancelation proves to be a better lesson for the
student in the long run than the instructional flight.

Yeah, right. And I'm sure the guy who took time off work, drove to the
airport, and then had you cancel on him 'cos the bloody compass card
was ilegible was really pleased with the lesson you taught him. It's up
to you how you teach, but I'm glad my CFI had a more realistic
attitude.........